Seriously. If you want to draw, stick with MyPaint and the N900. It's currently tops in regards to drawing at the moment. And due to the capacitive screens now doing pressure(s) and/or varying levels yet - there's a "beta" capacitive pen that's not in production yet that does levels quite well - I'd say go that above route.

And ignore the naysayers. Jailbreaking killing your warranty, restore that mofo and typically Apple is none the wiser.