Hey Bret,
I've seen a few try and do it with Javascript. The results however, from my point of view are not totally successful.
http://www.curvycorners.net/
for instance doesn't exactly work as I would want it to work in IE. It leaves a little bit of artifacting when viewing it in certain versions of IE7.
My method has been a table method. As I like tables =o
A 3x3 table with the content in the middle and scalable and the corner bg images in the corners. The top, bottom and sides with borders possibly. This method seemed to work pretty good for me. Though I know tables are a faux pas.
Abraham