While there are sketching/note taking apps, like
Penultimate and styluses (styli?) that work with the iPad like
the Pogo , you should realize that iOS is really not a note-taking/detail sketching oriented OS, in terms of pen/stylus input support. I don't think that focus will change either.
I think a mobile OS written directly for pen input might be a better long term investment. The coming Windows tablet OS, and possibly HP's WebOS (there have been rumors of stylus support in the past) will be a better bet.