If you are to go with the border-radius solution, there is this awesome website to generate the css that will make it work for safari/chrome/FF.
Anyway, I think your design should not depend on the rounded corner, and if you look at Twitter, they just say F**** to IE and opera users. Rounded corners is a nice to have, and I'm personally ok keeping this for the cool users who don't use IE :).
Now of course it's not the opinion of the clients.
Here is the link : http://border-radius.com/