IE will have CSS support for ie 9 release, which isent that far into the future.
They announced that time from ie 7 to 8 was a bit long, so the ie9 release should
be quite close..
But until ie9 there is no rounded corners by default in ie.
Quote:
IE9 will offer CSS3 selectors and rounded corners (it’s about time). JavaScript performance will also be improved.
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just google for it

IE9 is a major release.. so hopeing for the best ofcourse.
EDIT : link to a good article on features -
http://blogs.msdn.com/ie/ <--Scroll down.
EDIT 2 : if alot of your users are IE8 users, than you can force rounded cornors
instructions