The iPhone doesn't support multi tasking. If you chat, you can't do anything else. And if you do anything else, you get logged out of chat - makes it not really an Internet device - more like a glitzy one-trick-pony.

Android at least supports background "services" and chat programs take advantage of that.

The Internet Tablet is basically full-fledged Linux PC, so no limitations (although maybe also the least polished of these three)

Martin