it varies by handset. my mytouch 3g took a lengthy process involving 2 microsd cards, a hex editor, submitting some id numbers specific to my phone/microsd cards to some random website and getting emailed a "gold" file, and reflashing various files. my nexus one took all of going into the settings and unlocking it, then reflashing a cyanogen firmware that had been downloaded to the phone.

i would imagine that if at&t is locking down this particular phone, they are (or have instructed motorola to) lock down the firmware reflashing process.

If you get the phone from Google or T-Mobile it is trivial to root them and install your own firmware.

Hopefully Motorola won't lock down the bootloader too badly on AT&T.