I'd love this feature Sometimes when ppl have annoyed me, I come online later and send them sounds like a mad man... and then, in the end, it seems they don't have MsgPlus!... man, that sux
Another way this could be implemented is by changing the "credit card" warning at the top to something like "this contact has plus installed" or some thing similar.
The only limitation I can see with this is if you have multiple contacts in one conversation.
you know how there was talk about right clickin on the user to ping them to find out whether or not they have plus installed? well this would be nice if u could do the same, only ping them to see whether or not the contact has blocked you somehow, maybe using something similar to mess.be's Triople-extened block checker technology....that would be sweet