MD
The 3GS with the new bootroms was recently sw JB'd and it's untethered. See link for details. If you get tired of the phonebook SIM - enjoy.

iclarified.com/entry/index.php?enid=12132

limerain doesn't do carrier unlock but it does do hactivation (ie, it will activate your phone without using itunes).

If you want to keep a carrier unlock don't install 4.1 and then limerain. you need to wait for pwnage tool or snowbreeze that incorporates the new exploit so that you can keep your carrier unlocked baseband and install a custom firmware of 4.1

If you have a factory unlocked iphone then you can just upgrade to 4.1 and use limerain.