Skype 5 for Mac intentionally blocks Flash and other programs from using your webcam
The crazy thing is that you have to uninstall Skype 5 before using your webcam with other programs, including Flash applets on websites. It almost makes sense that Skype would reserve your webcam when it's running -- but even when it isn't? As the commenters on Hacker News note, the Skype for Mac support forum isn't the happiest place on the interwebs at the moment.
To top things off, Skype has known about the issue since the Skype 5 for Mac beta in November. Skype developer Beom Soo Park even admits that it's a feature, not a bug. There's no explanation for the very, very odd functionality, and no ETA for a fix.
[Just as this post published, the Skype bug tracker has gone offline. Hopefully it's just temporary.]