Why is iTunes only syncing some of the pictures (seemingly randomly) in each Aperture Event to my iPad?
Well, two trips to the genius bar couldn't solve it ("you need to start a new library from scratch and retag your 15,000 photos" - WHAT?!?!), but I dug around and experimented and finally came up with the answer:

iTunes won't sync pictures without up-to-date previews; so, if they stop auto-generating for some reason, you have to force it to update previews for the pictures that aren't syncing.


You may want to go to iTunes and select "Sync all Photos" and note the number of photos that appears in the upper right of the window, so you can make sure that it's higher after this process.
Go to Aperture and select "Photos" up near the top of your library, in order to select your whole library of pics.
In the "Photos" menu up top, you should see "update previews for library".  (If you're not seeing it, you probably skipped step 2, or selected an individual photo, in which case, it will be replaced with "update preview".  You want to do it for everything, so be sure to highlight "photos" first.
That may take some time, depending how many you were missing, but once it's done, go back to iPhoto, and uncheck "sync all photos" and then recheck it.  That should force it to recount the pics, and you should now see a number that matches your library (less any in a format that iTunes can't handle.)