Another option, if you're really serious about it (i.e. can spend some cash) is to use After Effects and a plug-in called real-shadows. Works fantastic.

You simply render your animation out as an uncompressed-RGB AVI (with alpha) or a image sequence (TIF, or TGA -- again, with alpha) import your animation and the background footage into AE, apply Real-Shadows, and it will use the alpha channel to create a shadow. You can then manipulate the shadow any way you want.