Here we go again:

http://android.clients.google.com/packages/passion/signed-passion-FRF91-from-FRF85B.db99fdf1.zip (1MB)

You must have FRF85B installed to upgrade. Presumably, ANY Nexus One (AT&T or T-Mobile, etc) that has FRF85B can use this upgrade

See our post about FRF50 for instructions on doing a manual upgrade.

Source: Google, XDA