I've been able to successfully view all streaming videos since I bought my macbook pro. Suddenly, I can't watch some online videos, such as all youtube videos. The quicktime symbol appears with a question mark. The MIME settings seem to be fine. Any idea how I can fix this?

Rumor has it that YouTube just switched to H.264. Is your QuickTime install up to date? Double check that you have the latest and greatest QuickTime (version 7.1.6 by my reckoning) and see if that fixes it. Also, what browser are you using? It's possible that something got weird about its communication with QT.

I do indeed have version 7.1.6. I'm using safari...

Weird... now, I realized I was totally wrong about YouTube switching to H.264. They're doing that eventually, for portables, and not right now... so if the YouTube clips are showing as QT question mark icons, maybe Flash has been confused into thinking it should play through QuickTime? Try reinstalling Flash, or check the MIME settings for that maybe?