I did a quick sample for you in the project below. The trick is to duplicate your source and change the bottom copy only have shadows. Have a light set to make its shadow where you want it.
Then on the top copy, shine another light that doesn’t cast shadows. Then parent the bottom copy to the top (I think I forgot to do that).
Hope this helps.
3696_lightandshadows.aep.zip
Mac Pro 2 x 3 GHz 6-Core Intel Xeon
32GB Memory
Dual-channel 4Gb Fibre Channel PCI Express card
Dell Display (23″ flat panel)
ATI Radeon HD 5770
AJA Kona LSi SD/HD capture card
Rourke 16 TB
Flanders 2460
SOFTWARE
Mac OS X 10.6.5
FCP 7
After Effects CS5
Boris Continuum
Sapphire Plug Ins
All Trapcode Plugs
Zaxwerks Invig