How to look up your contacts and see who can I talk with iMessage?
The feature you are looking for does not exist in iOS 5.0. I guess the reasoning is that you just want to send a message to your contacts and the device will send it via the appropriate route, favouring iMessage where it is available.

The closest you can get to seeing who has iMessage available without visiting each contact's card individually is if you go to send a new message, type 'a' in the To: field and you'll get a phone-number drop down. As you scroll, those contacts with iMessage available will have a little blue speech bubble to the right of their name and number (or email address).



Other that that, when you're sending a message...

If the Contact's Name and the Send button are green, then they don't have iMessage available and you'll send them an SMS (or MMS).



If the Contact's Name and the Send button are blue, then they do have iMessage available and you'll use that.