This thread helped me:
http://forums.omnigroup.com/showthre...1776#post41776
So, long story short, the "OmniFocus.ofocus" file on iDisk was corrupted somehow. I renamed it "OLD" and had my desktop app sync again. The desktop app then created a new database file (it's my most up-to-date version).
I then reset the database on the iPhone. Then I sync'd the iPhone to the newly created database -- which had been giving me that transaction error for days -- and it sync'd right away.