well, so far there are two solutions: purplera1n from geogot and redsn0w from the iphone-dev team.
- browse to http://blog.iphone-dev.org/post/126908912/redsn0w-in-june
- read the text
- do the magic!
works with Windows XP, Windows 7 and also Mac OS X!
- download purplera1n from here.
- make sure you have iTunes 8.2 installed (latest version so far)
- connect your iPhone 3Gs to your computer
- open iTunes
- run purplera1n, click "make it ra1n"
(it takes a while, your iPhone will reboot and come up with a different logo, after a while you should get to your normal home screen)
- run the new App Freeze from your iPhone to install Cydia
- reboot your iPhone
- use Cydia and have fun!
side note: if purplera1n crashes you are most likely running a non English version of Windows, this might cause problems. For me it worked with Vista 64 bit in English and German. Also there is a new version of purplera1n which doesn´t have problems with international versions of Windows, so if you run into this problems, try to redownload purplera1n from the website above!
Remember: jailbreaking / unlocking your device might void your warranty! You do that on your own risk, if your iPhone breaks, we are not resposible for that!