For something like random IC chatter, I don't usually have a problem with people who play from their cell phones. However, my one pet peeve is that not all messages go through and it usually leads to some sort of misunderstanding, especially if it turns into a serious discussion. Also the character limit can drive me crazy if it goes beyond the usual hey, how are you's. As for those who scene via mobile in short sentences, I'll never understand it and I refuse, it isn't worth it.
Personally I'd never sign on a phone for role play, because if I'm not on the computer it means I'm doing other things, but if others want to, all the power to them.