Andrew Bayer pointed out that he was getting this message when trying to load RFB in IE 6 for Windows:
Error: Expected ')'
It turns out the culprit was my half-assed attempted to install ThinkBlank's metalinker code into this page. In so doing, I apparently included an extra closing parenthesis (that's what happens when you cut and paste code without any idea about how it works or what it's supposed to say).
I've cut the offending extra parenthesis, which I hope will deal with the error, though I'll remove the code entirely if it continues to give people problems, especially as it's never worked they way it was supposed to (due again, no doubt, to my flawed installation).