I have three techniques I pick from when I need to employ rounded corners:
1) Totally fake it.
2) Put relative positioning on the element, and put four absolutely positioned div’s inside of it each set to each of the corners that have their respective rounded corner in it.
3) Use a jQuery plugin (there are several available, just Google around)
You can use use jQuery for #2 as well, if you want a more home-grown solution. I talk about doing that in this article a bit.