I also have itunes 7.6 and an unlocked 2G. I synced it yesterday and nothing became relocked so it's all good.
What service provider are you using? I use ATT, but I recently went to Thailand so I got my iphone unlocked and used a Thai sim card. When I plugged my iphone in my laptop using the Thai sim, itunes wouldn't detect it. But itunes will detect my phone if I use the ATT sim (thus syncing did not relock the phone)
Let me know if you also have this problem when connecting your iphone to itunes (using a non-ATT sim)