WARNING: I am not responsible for any issues or anything related to the matter of corrupting/messing up/destroying, or as i say, ****ing up your device!! Try this at your own risk!!
This is the step-by-step of EXACTLY how i did my JB on my 3Gs and 2G:
(It worked flawlessly on 5 other iphones to be exact)
1-Create a folder on your desktop and name it blackra1n.
2-DL 'blackra1n' from GeoHot and save it in the folder you created on your desktop.
3-Restore your phone to factory settings, update your firmware to 3.1.2 via iTunes 9, and if you are asked to update settings, do so(mines did).
4-Keep your phone connected to your computer at all times and keep itunes open (sounds like a dumb reminder, but i wouldnt remind if it hasnt happened
).
5-Click on the blackra1n file you saved in your blackra1n folder on your desktop(after opening, you may see a 'text' doc appear, dont delete, its just a log
).
6-Click 'Make it Ra1n'(Get ready to enjoy the show, your almost done).
7-You will see a picture of GeoHot in a few moments meaning you are in JB recovery mode.
8-blackra1n will install itself then your phone will reboot itself.
9-Once your phone reboots, you will be JBed and you will see a blackra1n app on your phone.
NOTE: It is recommended that you have a wi-fi connection for this next step but as long as you have a STRONG signal, you should be fine(I had a good signal and had no issues).
10-Click on the blackra1n app on your iphone
11-Then, DL Cydia and Rock and let them install themselves(When i DL'ed Icy, it messed up my phone everytime)
12-If you reached this far...CONGRATULATIONS, you are JBen!! Just open Cydia and Rock, one at a time, and let them organize themselves and u will be ready to roll!!