Another odd thing is that the css files of both plugins show the html page source when I use Firefox's css editor, despite the fact that I've checked and both css files are where they should be. Which is odd, too.
I deactivated all my plugins, and then reactivated the one plugin, and the problem persisted. In fact, it's happening with TWO plugins that were working just fine before, Social Media Widget and Sociable for WordPress 3.0 (which now produces the ugly list below each post rather than the eye-pleasing social media buttons were all know and love). I deleted and reinstalled said plugins, but no dice.
Needless to say, it's a bit perplexing.