For what it's worth, I purchased a 3GS a few days ago and it came with 3.1.2. When activating, I DID NOT have to update to 3.1.3. iTunes asked me if I wanted to but never was I automatically updated or forced to do it.
Also, I have since synced my photos using iTunes and I'm still running 3.1.2 (and intend to keep it that way).