I sort of called it when I saw the STARK earlier on, but I didn't expect it in this film.
I know what you mean about IM3 being busy, but they just seemed to make an obvious show of the two leaving together. Obviously it doesn't mean much, but I like to think that if Hulk doesn't get his own film (which seems unlikely) it makes sense they'd want him to show up somewhere.
I think Cap will deal with Bucky coming back, and as such I feel S.H.I.E.L.D might be present in that. Leaving Thor to deal with Loki.
Basicaly now there's this shared universe, I think they'll want to make the most of it.
If he is in it, then I expect it to be a small cameo.
Have they announced a Captain America 2? I'd like to see one, and have it go into more depth with the 'man out of time' thing, it was mainly played for laughs in this movie, but you could do a lot more with it.