What if I jailbreak first.

[yonathan]

Ok, so I have a new iPhone 3GS at 3.1.3. I got it from another carrier (I have a working SIM) and I want to use it on my second carrier.

I can activate it with the SIM card I mentioned first and then use Spirit.
Now, is there any way I could "hacktivate" it after jailbreaking so that I can use it with my "non-compatible" SIM card?

[i.Annie]

Jailbreaking is technically hacktivating so...no. You want to unlock the phone to use another carrier's SIM card. If your baseband is 5.12.01 then you cannot do it. If it is 5.11.07 or below, then you may do so with Blacksn0w (anything below the above mentioned uses ultrasn0w).