Cohdoo makes a nice iPhone UI sketching product called Phone Doos. They are 6.8" by 3.5" dry erase iPhone surfaces with faint grid lines and magnetic backing. A pack contains 10 Phone Doos.

I like that these gives you a surface to work on that can be re-arranged easily on a table and presented on a metallic wall. They have some good screenshots using them arranged on a whiteboard with flow arrows pointing between them, which is another thing that's probably much easier with these as opposed to using paper. Dry-erase also saves paper, which is nice. You'll just have to photograph your work, or make sure you have enough of them to keep making more screens.

The Cohdoo guys have web browser magnets planned as well. I'm looking forward to getting my hands on those to review as well.