[Rebirthpcg]

I wanted to know if it is possible to insert the Straight Talk micro sim card, and activate the $45 unlimited service on the jailbroken iPhone 4

[Massie]

Is it an AT&T iPhone? If so it should be easy—no jb or unlock required.
Straight Talk SIM: The BFF of a Galaxy Nexus or iPhone — Mobile Technology News

Actually thinking of doing this next month when my ip4 contract ends.

[Rebirthpcg]

Yes, its an AT&T iPhone 4, but I don't know the proper steps to enable the unlimited $45 a month Straight Talk service.

[Massie]

Just buy the SIM and follow the steps here: Welcome | Straight Talk

[rprather]

There's also a lot of good info in the comments here: Straight Talk SIM Card + iPhone 4S = $45 Unlimited Prepaid Plan » My Money Blog

I've had good luck so far with my iPhone 4S (AT&T, not unlocked) on Straight Talk.