I'm running a F17 KDE X64 machine. And I've noticed this issue over the last few kernel updates. I'm not sure that it's the kernel that's the problem, though.
Basically, if I plug in my headphones and watch a video/listen to music, everything is okay. But once I remove my headphones and want to play something off of my laptop speakers, there's no sound, unless I plug in my headphones again then I can hear everything perfectly (through the headphones, ofcourse.)
This isn't a problem with the laptop speakers, because it seems to work okay on a reboot. But logging out and logging back in doesn't seem to work.