before this version, everything was fine too. i made the Update about 20 mins ago.
I am not using it at all but to check my side i have installed all browsers
Just an educated guess.. But you know there is a compatibility mode in IE9 and sometimes it switches to this mode automatically (for whatever reason, it seemed to be very buggy in the beta).
Other than that, I'm pretty sure IE9 does support border-radius.
I've got all the vB4 rounded corners and drop shadows working in IE9.
Of course, I had to add them myself, since vB4 only has the moz and webkit variants. And incidentally the next version of Chrome doesn't use the webkit variants, it uses the "proper" ones, so vB4's rounded corners and shadows won't work in that either.
I'll be using images instead of the border radius property so i'm sure mine will display in ie9 there displaying okay in ie8. I would have rather used the border-radius but ie has always been in pain whenever styling.