This is new issue (for me) under F13 Beta. I've been running Fedora since Core 6 and never encountered anything quite like it.
Anyway, sound works perfectly, normally (didn't even have to hamstring pulseaudio), but upon plugging in the headphones, the speakers continue to play unabashed. A bit of googling as proved rather unhelpful (the alsa and pulseaudio documentations are a bit weak). I suspect the issue stems from a a driver issue, though I've yet to find it.
For the record, I'm running an Asus eeePC 1000he on Fedora 13 Beta, installed from a KDE live cd.