Paper from FiftyThree[1] is very good. Omnigraffle is a useful tool to, but pricey. Photoshop for ipad looks promising too. Matt Gemmell recently wrote a blog post about using the iPad from a UX designers point of view[2].

[1]http://www.fiftythree.com/paper [2]http://mattgemmell.com/2012/05/02/ipad-productivity-apps/