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.
Should also mention that I've checked his javascript/security/popup settings and they match mine. Plus he can see all kinds of similar popup windows (greyed out background) for Lightbox effects, etc, including the Add Media popups (this made me think it might be a Tiny MCE problem, as I've heard others suggest on the web, but it's odd then that I can't replicate it...)