Is it poss able to to read someones IM messages if you share the same wireless internet but not computer?

Actually, encryption is an option for some IM clients. So, if you don't have SSL turned on in your client, the connection is in the clear. I think someone on the same network segment that you are on would, in fact, be able to see your IM traffic with a network yzer (eg: wireshark). I'd need to do some testing to see how easy it would be to read the messages going back and forth, but it is possible.