When I sync my iPad, a random selection from each event is there after I sync. Some events seem to have all of them, some have only one (of hundreds), some have a minority percentage.
And there's no rhyme or reason - it's not the first, last, edited ones, or any other category I can think of.
I've tried rebuilding Aperture's database, turning photo syncing off, syncing, and then turning it on again, restarting, all to no avail.
I do get the "some of your pictures couldn't be sync because they can't be dispayed on your iPad, but I always do, and can't fathom that's the issue - they're standard, non-giant jpegs.