I used combustion v3 to do the comp at the end of this super 8 short:
http://www.youtube.com/watch?v=rdvi-1TsW6o
I didn't even shoot it on green screen, just a plain wall. I used the "draw matte" tool and the tracker as well as a couple travelling mattes to comp the head on to the toddler's body.
The film stock used was Ektachrome 64T and it was transferred to dvcpro, although dv would have worked just as well.
If you want to do something similar but don't want to buy combustion, you can get a free 30-day trial demo of Shake:
http://www.apple.com/shake/trial/