Well, first of all, it isn't called "jailbreaking"
2nd, you'll need someone to do with it a hardware flasher.
You'll need to solder a bunch of wires to the NAND.
I unfortunately don't know anyone that can do it but it can be done yes ;]
"jailbreaking" should be associated with iphones only imho.
Anyway, doesn't matter much tho since you'll need to downgrade your console.
Only way possible is with a hardware flasher, so best thing is to bring it to a modshop ;]
Devin used to offer downgrades, but that's for NOR consoles only.
seeing you got a launchday console (possibly also backward compatible) it's highly likely to be a NAND console, which needs soldering.
You could send him a PM and ask, but I don't think he still does it.
Hummm, you need an iHAS B model.
I think newegg has them (if you live in the states)
otherwise, have a look through this list: http://tinyurl.com/ihasdrive and search for one that is compatible ;]
You'll need a free moles plug in your PC to hook it up tho.
Or get a USB to molex adapter or whatever.