For us experimental electronic musicians; long before the iPhone was out, some research students had made a touch sensitive table-top instrument that, when certain objects were placed on it, would react with sounds. Way back, when I used to work for M-Audio, I had contacted these guys to try and license the technology from them, but they politely (and smartly) said "No thanks." Man I wanted one so bad! They did build one for Bjork however which she used on tour... Now, they have finally made an iPhone/iPad app version that essentially does the same thing (but with virtual objects of course!) I've been waiting for this since before the iPhone even existed! Here's a screenshot from my first session with it. You supposedly can upload your own sounds/samples as well, but I haven't figured that out yet.