Been scouring the net for others with this problem. My client is getting a blank popup on both IE8 and FF3.5 (both Windows) when he tries the following Visual Editor buttons (and we need the Visual Editor):
Add a Link
Paste from Word
We're using WordPress MU 2.9.2.
When I log in with IE8 or FF3.5 on an XP or a Vista machine, I can see things just fine.