Need to relock unlocked iPhone 3G to work with AT&T?

My wife intends to unlock her 3G while in China so she can use one of their carrier's networks. We currently have an active AT&T plan in the U.S., which we will keep active while she's gone. When she returns, can we simply take out the Chinese SIM from the unlocked phone and put the original AT&T SIM back in and have everything work normally with AT&T? What will happen when we connect her unlocked phone to iTunes and attempt a sync and/or firmware update? My guess is that a firmware/OS update will completely erase/restore the phone and *relock* it. I'd like to avoid relocking it, if possible, because we'd just have to unlock it again next time we're in China. It'd be great if we could hot-swap SIM cards on the unlocked phone......

One thing I think is possible (and correct me if I'm wrong) is that we can indeed just put the AT&T SIM back in, and it should work fine with AT&T and iTunes (can still connect to iTunes and sync). But we *should not* do any firmware updates (which would lock the phone again). Should we also avoid OS updates? Or are those ok?

Thanks for your time. I'm open to questions if anyone needs clarification.

Unfortunately, the moderators here will lock this post up as soon as they see it, but if I were you, I would just avoid firmware updates once it's unlocked. It can't be locked back up unless you update the firmware on it via iTunes