I frequently switch between my external computer speakers and my headphones. Repeatedly plugging and unplugging my headphones started wearing out my headphone jack, so I hooked up an iMic to a USB port to give myself a second headphone jack. This allows switching between them in software through the Sound preference pane, which is more convenient, but still takes too long. So the next step is automating the switch. This thread on Mac OS X Hints has people discussing various ways to do it. The easiest GUI way to do the switch on Snow Leopard is just option-clicking the menu bar volume control and selecting the new output device (on Leopard, you can do effectively the same by installing SoundSource).
Sounds stupid but I’m a Mac noobie so just putting this up there. I constantly switch between headphones and speakers and this was annoying the shit out of me.
its Mac simple (which makes me feel dumber!): Option+ then select new output. Duh!