Site versions on different servers producing different output, but only in IE9

Super weird: I just set up a server and moved files for a site off our testbed onto the new server. Old testbed server is running off CentOS5 (php 5.2.16). New server we just moved to is running CentOS6 (php 5.3.3). Both have the jquery updates installed (1.3.2). Both have identical templates and .css files.

The old site has a Nice Menu with some lightweight animation built in and it works totally fine in all browsers. All the work is done via .css, nothing done to the module, no custom anything beyond the .css. But on the new, live site, the menu dropdowns don't work and the menu is basically a hash -- in IE9. It is perfectly fine on the new server in Chrome, Firefox, Opera and Safari. It's just IE9 and it's just on the new server.

Any ideas? It is weird and confusing and frustrating because there's literally no difference between sites other than the php version, as far as I can tell.

There's some other stuff that's also not showing up on IE9 on the new server that may be worth mentioning, in particular border-radius settings and linear gradients (in IE, stuff like filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#5087C7', endColorstr='#25408F');). Again, those show fine on the old server in IE9, but not the new one.

Super weird. Looking for ideas. Thank you!

p.