I'm actually really pretty excited about this. However, from a UI point of view, rather than selecting a list and doing a manual File -> Export, wouldn't it make more sense to just treat iTunes as a handheld? Then we would be able to use all of the existing handheld options, conversion settings (cache and whatnot), and an existing UI system. Perhaps this is just step one and you are heading down that road, but...
It would be REALLY nice if I could just set MC up with a special "iTunes" handheld. I'm envisioning a system very much like the current "generic handheld" system (where we can sync to a folder on our hard drive) using the pre-selected Smartlists and manual Playlists chosen in the MC Handheld settings. The important thing would be that the presets for converting videos with FFMPEG, building a conversion cache, resizing photos, and the rest of MC's awesome Handheld options would be set to work well with iTunes (this is possible now, but you have to manually hack the FFMPEG.xml file for video support). Then, when you click "Sync" it would open iTunes and update the iTunes Library to match the MC Sync list (throwing out file entries that are now "missing" and whatnot). I'm, personally, not really concerned with capturing playcounts and ratings back from iTunes in the process, but others might be interested in that as well if possible.
That way, we can use MC on our computer for our "master media library", and then maintain the iTunes library separately, but via MC. The iTunes library would contain ONLY those files that you want synced to your phone (which makes iTunes' stupid sync support easier to deal with anyway, you just set it to sync everything). I suspect many users of MC have libraries that are far too large to even consider syncing over to even a 64GB iPhone, so this is probably an ideal way to handle it. I already accomplish this manually by "syncing" MC to a folder, and then running a separate iTunes folder sync program that makes the iTunes library match this folder. If MC could handle this second step for me, it would make using MC with my iPhone (and, more importantly, my wife's iPhone) a LOT less painful.