Thanks everyone. A while back I got a few CDs of digital backgrounds along with a starter kit for blue screen, so I have plenty of options to play with.
As an aside, it looks like there is a blue screen / green screen plugin for Aperture II, which might just tempt me into trying out Aperture again.
Wonderful idea, Jim. For this one, I'd place the beauty lower in the frame, as not to be to close to the sign, and try a very light drop shadow for a more realistic look :)