I guess there are 3 technological hurdles to a (good) stylus on iPad;
1) pressure sensitivity and input precision on a rigid glass panel -
This is probably the only genuine hurdle (i.e. needs major R&D) to a pen filled iPad life - I'm sure there are many ways to handle it, with cameras, active pens, etc - but Apple have to test and develop a system that brings no compromises to the aesthetic and functionality of the device and doesn't undermine the primary input method - fingers.
2) the distance between the pixel and the pen nib -
integrating the panel, touch matrix and glass into a thinner and tighter sandwich helps this. could be problematic with the extra tech needed to support problem 1
3) handwriting recognition technology -
I've found the Macs handwriting recognition to be pretty crap when using a Cintiq, but then again so is it's voice recognition, and Siri has proven so much better than what OSX has to offer.
If Apple nail all three of those, we can eagerly look forward to an iPad that supports a $70 optional pen accessory, which can be stored in a $40 optional smart cover with a pen pocket.