Switching between headphones and built-in speakers seems to work in both 32-bit and 64-bit Snow Leopard on my MacBookPro (5,2) 17".

Headphones are automatically detected when plugged-in and the speakers are turned (and vice versa).

I am using really cheap, crappy headphones too, in fact I might just trash them right now. They have no third channel for a microphone.

It sounds like you upgraded from Leopard, you might want to try a clean Snow Leopard install instead (which is usually a good idea anyways).

Alternatively, you may want to determine which kext is responsible for audio on system (AppleHDA.kext?) and try replacing it. If you like I can post a copies of kexts from my 10.6 installation.

Good luck!