I've been doing this since I got my iPad 1. Got a lot of laughs at work when I try to do something by touching the screen of my laptop. We had a programming course last week and I tried it on the monitor of the PC that I was assigned. The instructor got a good laugh out of that as he disliked anything Apple.
When I was lining up for the iPad 2 in an Australian retailer, we were all queued up around the apple section. The woman next to me proceeded to reach out of the queue and tap on something on the 21.5" iMac. Samywamy10 resisted the urge to laugh.
Why she was lining up for an iPad was beyond me...
I was looking for a particular song on my old 5G iPod and kept jabbing at the screen before I remembered, oh, yeah, this thing has a CLICK WHEEL, it's not my iPhone or my iPad!