Sorry if this is a repost. Didn't see it anywhere
Found on msftguy's blog
STOP if you have a new bootrom (week 40+, tethered only 3.1.2 JB etc). Here's how to check bootrom ver
- your hardware is iPhone 3GS with OLD BOOTROM
- you HAVE 3.1.3 SHSH (**)
- you DON'T have 3.1.2 SHSH (otherwise, just use blackra1n/redsn0w).
- you WANT iOS4/JB

This tool can be used to flash pwned nor files (containing LLB exploit) on the phone running Spirit JB (script has hardcoded offsets for 3.1.3 3GS).
Mostly iPhone hacking: OLD BOOTROM + Spirit => 4.0 JB
Instructions and files at link.