Just read that x-menfilms.net has said that they are 100% sure of a cameo by the following characters:
SPOILER ALERT!:
Omega Red and Gauntlet. I don't actually know who Gauntlet is, as I've not been keeping up on things too well for awhile, but Omega Red could be interesting. There is specualtion on their site, that the Omega Red scene could lead into the Wolverine solo movie that's coming up..........