iDector is a real easy to use application to tell wheter or not your iPhone 3Gs contains the New Bootrom or the Old bootrom.
Bootrom
Posted inHOWTO / NEWS
DevTeam: iPhone 3Gs With New iBoot NOT Jailbreak-Proof
Eric McDonald, a member of the DevTeam, said in a telephone interview with wired.com that there is nothing to fear about, because even on the “new iPhone 3Gs” we will be able to run the Jailbreak.