Mark Ruffalo has told interviewers that he was signed on for The Avengers sequel, noting that "unless they have a change of heart--and they have in the past--I'm there."  With a hit as big as The Avengers, and the fact that Robert Downey, Jr.’s contract may be up, reuniting The Avengers cast may not be a simple matter (see "Will Robert Downey, Jr. Be in 'Avengers 2'").  At least it appears that newcomer Ruffalo, whose Avengers role was his first for Marvel Studios, is signed on for the sequel.
Another HitFix story that appeared over the weekend reporting that Marvel Studios is negotiating with actor John C. Reilly (Talledega Nights) to portray Rhomann Dey in Guardians of the Galaxy remains unconfirmed.  Dey, as Marvel fans know, is the fictional head of Nova Corps, which could of course lead to Nova’s appearance in Marvel movies.  But according to HitFix report, Dey would serve a function in the Guardians movie akin to Agent Phil Coulson in The Avengers--working as a sort of go-between by coordinating with the Guardians and reporting to S.H.I.E.L.D. about their galactic adventures.  The unconfirmed HitFix story does have an odor of believability about it, since Reilly’s considerable comedic abilities as an actor would appear to fit in well with Guardians of the Galaxy director James Gunn’s quirky sense of humor (see "Marvel Eyeing James Gunn to Direct 'Guardians'").