Have Ubuntu developers considered adding antialas to a window manager(metacity) in Ubuntu 11.04? If there is coming Unity and Compiz is needed, why not to add this feature to corners of windows manager?
Installing Emerald solves this problem. It bothered me too so I've done it. Here's a screenshot:
Details on how to install it if you're using Natty are in this WebUpd8 article.
What you're describing is tracked at these two bug reports:
From a comment by a Metacity developer in the GNOME bug:
Also, I believe Roland is incorrect; Compiz's Bicubic Filter plugin doesn't anti-alias window corners.