Apple is now shipping the iPhone 3GS with a new Bootrom that is not vulnerable to the 24kpwn exploit.
The iBoot-359.3.2 started to ship last week. A screenshot posted by Mathieulh shows the new version number.
MuscleNerd from the iPhone Dev-Team has confirmed that the loss of the 24kpwn exploit would mean a normal jailbreak would be impossible for the time being.
oh oh, from what @cpich3g + @Mathieulh have found so far, a normal untethered JB may be in jeopardy in brand new 3GS units
For the time being we would suggest locating old stock or refurbished units. Hopefully, the dev teams will be able to find another exploit for the new bootrom.