For the sake of a bit of unobtrusive scripting, I think it's worth helping the old browsers
I disagree, but we can here. I don't feel comfortable scripting all sorts of functionality into broken browsers like that. Square corners isn't a dropdown menu, as far as importance.
I'm aware of all the cssPIE stuff for IE and the scripts that help IE recognise HTML5... waste of time unless someone (boss) tells you this is the Only Way Or Else.