blank site in Safari and IE

Wordpress 2.9.2.
Atahaulpa theme 3.4.6

My site is blank, except for the tiled background image, in Safari 4.0.4 Mac and IE in XP.
Looks fine in Firefox Mac and XP.
When I activate the default theme, everything looks good again in all browsers.
In an attempt to find the problem, I've removed customizations here-and-there, saved, viewed, but no luck.

I wonder if anybody has experienced this before and can point me in the right direction.

Thanks!
Cway