PC users can't open iMovie .m4v file

You could always point out that anyone whould really wants to watch it could download and install the free QT 7 Player in much the same way that many Mac users would have to download and install either a WMV Player or a WMV component for their QT Player.
You realize, of course, things will likely get worse once you start "customizing" your exports of each recipient? My Sister only wants WMV 9 ($50 for Microsoft "approved" bi-directional components), my brother-in-law "prefers" DivX 6 minimum ($20 for the components), my nieces are into YouTube (posted directly from iMovie '08 for the price of iLife '08) and want everything posted there, and my Aunt (whose computer is more than a decade old) only wants MPEG-1 (which I used to make with iVCD (don't remember what that used to cost or Ti Toast 9 at $80) and of course there is always Visual Hub (at just under $24) for all of the others who will settle for older AVI or WMV files. You can also try exporting some of the "less recent" or "older" codecs like MPEG-4/AAC, Sorenson Video, Sorenson 3 Video, H.263, Cinepak, etc. There are dozens of audio and video codecs that can be combined in in hundreds of combination and placed in various file containers but the problem is you will probably never find a single compression format that really satisfies everyone all of the time. So before anyone can "draw you a map," you will have to make a decision as to "down which road you wish to travel."