The missing 720x480 iMovie 08 preset

Actually they remain 960x540 and 640x480 with respect to their "physical" matrix display on a "square" pixel device. While both are stored in a 720x480 (1.50:1) "rectangular" pixel data (i.e., "logical") matrix, this matrix could be "display decoded" to many different dimensions if the proper flags and algorithms were ever made available.
Sorry if I fail to see your point, but why do you want to store your files in a fixed 1.50:1 NTSC "logical" matrix but display them using their original square pixel "physical" matrix when they can be stored and displayed using the same "physical" matrix which, according to your settings, is not targeted for either DVD or DV transfer.

Actually, you are trading a slight loss in quality for your reduction in file size. Since you are not targeting devices historically used with rectangular display, I assume you are using the NTSC 720x480 storage matrix (i.e., max of 345,600 cells) to store the equivalent of 409,920 (853x480) pixels/frame worth of information. Since it appears your files are destined for viewing on a monitor or HDTV, playback by any player recognizing the anamorphic flag means the 480 pixel height will be held constant but that the width will be scaled from the 1.50:1 storage aspect to the 1.78: display aspect which does equate a slight loss in quality when compared to a one-to-one storage-display relationship.

I want to export 720x480 video, because that is the same as my source footage. I want to skip resizing to minimize quality loss.

Sorry, but this is not possible. iMovie '08 recognizes the anamorphic flag and scales your files to the correct square pixel equivalents for use within iMovie 08. Thus, when you later export your project to an NTSC standard, your are again "resizing" the file back to the 720x480 matrix for storage which is then once again rescaled to 853x480 for viewing. In effect, you have expanded your original file for editing, re-compressed it back to its original standard tossing out the newly interpolated data, and then re-interpolated the "missing" data for display. If that is your goal, then fine you have succeeded.
If storing/displaying the edited project at 853x480 creates a file larger than you desire, then the other alternative would be to hold the width of your target file at 720 (your original width), but scale the height to 405 (exact 16:9 aspect) or 404 (nearest "sub-block-16" element for more efficient encoding/decoding) which better renders the original quality with an even smaller file size but at the cost of a smaller display area. (I.e., your original 345,600 storage matrix has been reduced to 230,400 storage/display pixels/frame which you say you don't wish to do.
Basically, all I am trying to point out is that it is impossible for you "to skip resizing" since your file data has to be decompressed at least once in order to edit it and that your work flow actually requires a secondary resizing as part of the export "re-compression" phase. If your work flow "works for you" and you are satisfied with the results, then stick with it. Just don't think "you are getting anything for free" by using it.

Gee, that is truly amazing. My computer seems to work totally different that yours. On mine, the 720x480 data is actually mapped to 853x480 pixels for editing. Under your hypothesis, if a 720x480 anamorphic clip containing a 1-pixel width vertical line is imported to iMovie '08 it retains its 720x480 dimensions and is merely "scaled" for display purposes. If this is true, then I should be able to edit the clip by inserting another 1-pixel wide vertical line and it should appear at the same width as the first since each is 1-pixel wide and automatically mapped to the canvas at the same resolution. But this doesn't happen. The second line is narrower because the first line was not merely scaled but instead actually re-mapped to the 853x480 square pixel equivalent dimensions. Further, when the edited project is finally exported using your work flow, it is again mapped back to a 720x480 matrix retaining the relative differences in displayed line widths as you can easily see if you download/compare them at twice normal player size in this test file: 720x480dv.mov

It is obvious that video has to be decoded/uncompressed and encoded/compressed through the workflow. But I believe you are mistaken if you think compression and decompression equals resizing.

Don't believe I ever indicated that decompression alone equaled resizing. However, decompression and remapping does. Why don't you try a secondary experiment. Try taking one frame from your "Event" file and adding it to a project as a "still frame" sequence. Now "Control-Click" the just added sequence and select the "Reveal in Finder" context menu option. Double click the highlighted file and check its dimensions in the Preview "Inspector." Is it really 720x480?

Your original statements seemed to imply that you considered 720x480 aspect corrected content to be both qualitatively and materially equivalent to 853x480 created content which is stored as 853x480 content and then rendered as 853x480 content on a square pixel device but without the increase in file size associated with the larger native dimensions. Further you seemed to imply that the aspect adjusted file retained its original matrix structure during editing and was merely "scaled" as if seen through a lens for display purposes in iMovie '08 rather than being re-mapped to the encode square pixel work space indicated in your file format line -- i.e., "720x480 (853x480)." If I misunderstood your intent here, then I apologize. However, I still contend that your file is re-mapped for edit purposes and the edited content will undergo an applied transform to re-map it, as well as, any new/mixed content back to a 720x480 matrix using your work flow.

I just tried saving out an AIC codec quicktime file with the "Current" size. It saved out at 960x540 resolution. This doesn't tell us much, either. Does iMovie only work with 960x540 res internally?

Another experiment. Try resetting your "Project Properties" for your anamorphic content project to "4:3" and try your AIC export one more time. On my computer the "current" size selection changes from "960x540" to "640x480" pixels. Thus, it would appear that the "current" size of your project is controlled not by the content itself, but by the "project properties" setting. In fact, the "current" value for any project not 4:3 will be 960x540 -- to include 1080i content or 1440x1080 (1888x1062). Bug? Possibly. As to internal workings, see Steve's latest update.