Well I don't think Amazon's CPU is going to buckle over me syncing a few books not bought by them.
But you are right, it would probably just be easier to use the location numbers. I'm doing Amazon a favor, if I can find a good system, I'll buy a Kindle...
I wrote the other thread about the locking up. Apparently Amazon iphone app locks on TEXt REAd books, but not on proper MOBI books. So it will let you read non-amazon books... but the whole non-syncing does kind of suck.