Be careful with this. We've used it on a number of sites, and whilst overall it's beneficial, there are a number of weird things to watch out for.

For instance:

  - Don't add rounded corners to a <table> element - this will crash at least IE7 on XP
  - Don't try to rounded corners an element you've hidden with the "left offset" technique (position: absolute; left: -9999px;). This will crash at least IE7 on XP.

I think there a few others I have since erased from my brain due to IE-induced PTSD. YMMV.