yup, has to have cellular. which doesn't make sense to me as it's network data.. maybe it's a stupid policy by apple
the trick is those cross platform ones have to be installed on all your friends devices, i have whatsapp which some people have but it's not free so not everyone has it. I also have impro+ which connects to all the various im things like google talk, yahoo, icq etc but i don't really like the look and feel of the program, also have talkatone which connects to gtalk/googlevoice and in addition to messaging i use it to make free ld calls through google voice (which is what i use it for mainly). I like the way iMessage works, BBM seems to be thought out as much as iMessage has...