I've been seeing youtube videos stall badly in general when pulling the straight H.264 versions (as the iDevices do). Pretty bad though if local playback is also being impacted.

As a test, I checked iStat for running processes. It showed a number of 3rd party ones in there. After rebooting the phone, the only 3rd party process that came back on it's own was Skype. All others appear to be not auto restarted by the reboot. Keep in mind all apps will always show in the multitasking pane, unless you clear it manually. It's basically a big recent list for apps, running or not.

I was unaware iOS would restart 3rd party processes after a reboot. Handy for VoIP I suppose.