iphone 3G didnt use 24kpwn, it used the pwnage 2 exploit instead (which from what i understand is actually two different exploits; a tethered exploit and an untethered exploit the system is jailbroken with the tethered exploit, and it's made bootable with the untether part.
24kpwn is another exploit used to allow untethered boots, but it required the system to be jailbroken (via a tethered exploit) beforehand. This is why SHSHs were problematic in the early 3GS days; the tethered exploits were usually in iboot, which was always updated with new software. Thanks to limera1n, the 3GS (old bootrom) is now permanently jailbroken regardless of OS, much like the 2G and 3G
