Nice idea! The ability to communicate with others like this could lead to some cool things happening.
In addition to scheduling demonstrations or informal meetings, I can imagine people posting messages to the owner like "I have a spare joystick for this rare game, contact me at XXX if interested". I have gotten messages on my machines like this in the past ... usually a card set on the machine. Post-it notes would be better. The only concerns are that the notes be put in places that won't damage the machine (on glass or T-moulding might be best), or obscure the view for the player, or significantly cover up art detracting from the appearance of the machine. We supply low tack labels to exhibitors so they can put their name on the machine without having to scrape off stickers later (our labels peel right off). Post-it notes next to these labels is a great idea.
skierpage, want to own the idea? If you could put together a small sign to explain what to do with the Post-its, we can put them on the registration desk. I'll get you some raffle tickets or something as compensation.