Well… if you know the content will always be of a certain width and can guarantee that it will never be any taller than you containing div a single image solution would probably work. Or if you were willing to include an IE (6,7,8, and now 9 too; good grief :headbang:) specific stylesheets you could use CSS 3 for browsers that will support it, and only give the bummer to IE users. Because not all sites HAVE to look exactly the same in every browser.
Otherwise a sliding doors approach, or other multi image fix would be your best bet. I have a feeling that the best chance you have of getting it to look and work like you want is going to be sliding doors. You can even have the CSS, HTML and images all generated for you if you did a quick google search for rounded corners CSS, or something along those lines.