Q: use external isight camera instead of built-in isight

Is it possible to have an external isight camera be default instead of the built-in camera at the top of the monitor?  I'd like to use the external camera, which is more portable than the monitor, with such programs as Skype, but -(on Skype at least) I'm not given the option to choose anything except the built-in iSight cam.  Can the external iSight be the default cam?

Thanks.

Lee