Will flashing re-lock my unlocked phone I bought an unlocked android phone at Amazon and it works perfectly. This phone in particular doesn't have any OS updates I could do (Motorola Defy), so basically I am stock with Froyo unless I flash it with a custom ROM with an newer android version.
The phone used to be locked to T-Mobile's network, so my question is: since my phone is currently "network unlocked" if I flash it will it re-lock again?

I appreciate any help and guidance. Thank you.
No, flashing a ROM on your unlocked phone will not re-lock your device.  I have several unlocked phones which I flash ROMs on to without any issues.  The SIM code which operators use to limit what SIM cards will work in your device has nothing to do with the software running on that device.