Thanks for responding Nemi. I'm not sure what you're describing, \u005F did nothing for me in either browser. Anyway, after I made this post, I continued my research and eventually found my way into the CSS2 errata, which revealed that the underscore is supposed to be allowed in identifiers. Apparently this isn't (yet?) reflected in the W3 CSS validator though. As for the escape sequences, according to the spec, this is the syntax:
Third, backslash escapes allow authors to refer to characters they can't easily put in a document. In this case, the backslash is followed by at most six hexadecimal digits (0..9A..F), which stand for the ISO 10646 ([ISO10646]) character with that number. If a digit or letter follows the hexadecimal number, the end of the number needs to be made clear. There are two ways to do that:
1. with a space (or other whitespace character): "\26 B" ("&B")
2. by providing exactly 6 hexadecimal digits: "\000026B" ("&B")
Netscape 6 properly evaluates the escape sequence, IE 5 does not. But the literal underscore is allowed, thank god.