Are you trying to import from the camera itself or are you trying to import from a backup of the camera, where you have copied the entire file structure from the camera to your HD? In the video app forums, often people have copied only the MTS files from the camera thinking that's all they needed to keep and then find they can't import them. Judging by your posts and that you are following the instructions from Apple, I assume you are working with the entire file structure but I thought I'd mention this anyway, just in case. If you don't have the entire file structure, you can purchase Clipwrap to convert the files for import.
I hope someone with iPhoto 11 steps in here soon. I don't have it so I can't try out the AVCHD import feature.
I have the same problem with iPhoto 9.3's advertised ability to import AVCHD videos, and I also carefully followed Apple's instructions. (I am using the latest version of Mac OS X too; 10.7.4.) I get lots of "unreadable files" due to "unrecognized format", even with MTS files.
In addition, Apple's instructions imply that the camera should either mount and allow AVCHD movies to be imported in the usual way, or not mount at all, in which case Apple's instructions for copying appropriate folders should be followed. However, my Panasonic FZ150 mounted normally, but no AVCHD movies showed in the list to be imported.
I reported this to Apple via "Provide iPhoto Feedback" in the iPhoto menu.
Has anyone got this to work with the publicly-released version of iPhoto 9.3?
I have now discovered that, in my case at least, iPhoto 9.3 does not import all AVCHD formats, so I suggest you check what type of AVCHD you are trying to import.
iPhoto 9.3 import works for AVCHD Lite (720p) and AVCHD (1080i), but not for AVCVHD Progressive (1080p), which is what I was trying to import.
I informed Apple yesterday at email@example.com, (who had responded promptly to my original report of this problem), and asked them to clarify this in their website, in iPhoto 9.3 and in all relevant documents, until such time as Apple supports the full 1080p AVCHD.