We do the "moving sketches around" way of collaborating, and it works, but it tends to be a bit slow. There are some ways to get around the slowness: plasq's skitch makes it easy to share the things on my screen with other people, for example, and sometimes we just skype psds to each other. And I'll certainly keep using those tools.

The iPhone mockup tool I've created is a very specific tool for a very specific issue I have: I'm thinking about the layout of an iPhone screen and I want to quickly throw some ideas at somebody who's far away, and let them give live input. I don't even think that the mockup tool is the best solution for that specific problem, it's just the first idea I came up with :-)

I find it useful, but I can totally see how it's not for everyone.