iPhone Dev team has released jailbreaking tool redsn0w 0.9.5 for iPhone OS 4 beta, but some developers have reported SMS and 3G data failure issues.
Musclenerd, Dev team member, has now released redsn0w 0.9.5 beta 2 to fix these issues. In order to use this beta version, you will have to restore iPhone OS 4 beta firmware.
Musclenerd on Twitter -
For those 4.0b1 JB devs with SMS or data issues using redsn0w, the update at http://is.gd/buMgi
should fix (restore 1st!)
Download Redsn0w 0.9.5 Beta 2 for Mac
[We've removed the downloadable links from our site due to policy violation issues. Sorry for the inconvenience.]
You can follow the same steps given here to jailbreak your iPhone 3G.