How to unlock and root a Nexus One

The radio does not support AT&T's 3G frequencies. If you are fine with GPRS/EDGE, you could already be using it on AT&T.

You've always been able to. The nexus one was never _carrier_ locked. (But as mentioned, the different frequency that ATT uses for 3G is unsupported -- so just voice and EDGE data.)

This "unlock" is, as described, attaining root level permissions on the phone, thus the ability to do things in software that the stock firmware disallows.

Perhaps from the standpoint of at least a portion of the imagined target market, that's a feature rather than a deficit (aka more fun/interesting that way). ;-)