Word has come down that RDJ signed a deal with Disney/Marvel to return for Avengers 2 and 3. No word on Iron Man 4 at this time.
Downey was reportedly asking for 100 Million to reprise the character. Large portions of this were to be paid to his fellow Avengers co-stars to supplement their meager (in comparison) salaries. No word on how that breaks down, nor will it likely be made public. Good job Iron Man!