While I agree that it's nice to have a bottle of shampoo for those hair emergencies, I don't think buying a new bottle is necessary. The cocobetaine in the Burt's Bees should be sufficient enough to remove any silicone build-up.

Here's a good article from our own Curl Chemist on surfactants to look for:

What's the Scoop on Silicones? | NaturallyCurly.com
CG since 6/15/08!
Medium-Fine/Normal Porosity