My Pre2 is an unlocked Rogers
I used: Palm Pre/Pixi Unlock Instructions
(it required you connecting the phone in dev. mode to a computer)
Once unlocked, the phone should be useable for any service provider.
The instruction were simple (see link), and not had any problem at all.
I use it with Bell.
Before it was unlocked, it was showing (once the SIM card inserted) that I would see the Bell network but refused to connect. Once unlocked, turned it on and no more problem, it light up, connected and everything was great.
I checked with the company (that provided the unlock) an they assured me that once unlocked, it stays unlocked and all that matters is the SIM (provider).