Will the phone back in "locked" after upgrade?

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Hi all,

I have the X10 and i love it in everyway.

I got my X10 from A&T and I unlocked it to use on US T-mobile network. The phone is working flawlessly.
However, my question is will the phone lock back to AT&T network after I do upgrade it to 2.1 (I heard Sony and AT&T will put out a 2.1 together), OR will it remain unlocked for the rest of its lifetime now.

I am new, and appreciate all the help there is.

No one know this?? or Am I in the wrong forum?

Once the phone is unlocked it stays that way, unless you take it back to the A&T and ask them to lock the device again(but not the case here).
the update is released by SE and i think that it will only affect the rooted devices,meaning that if you rooted your device, you will need to unroot it again, when that will be possible again.
If i am wrong someone please correct me.