Archos have a similar thing with their andoid tablets whereby you can remote control them from an android phone. But that's their own proprietary closed solution.
I'm thinking of doing the same thing for when I get my R-pi (only I have android, not IPhone).
I think all it needs is some wrapping around the xdotool with a server that accepts mouse / keyboard commands to come from the client.
The client side would need an android / iphone app that provides a mouse/trackpad function and keyboard - more fully featured than the default ones provided. (I think this part would be the hardest, to be honest).
This would be really handy for websurfing on an HDTV from the sofa, without the need for bluetooth gear.
However, I don't have an IPhone, so I'm not sure could develop and debug the app for that platform.
Once I have a working prototype running on 2 linux machines, I'll post the code up at google code or somewhere like that.