This is a very slight modification of the Ambiance window decoration theme as found in Ubuntu Oneiric.
It uses the metacity v2 spec in order to define less rouned corners, which may feel a bit more squarish, but at least do not look pixelated. Just keep it for a couple of hours and you won't notice that it is slightly more squarish. Actually I think it blends much better with the rest of the Ambiance theme!!
Please note that this is the only way to avoid pixelated corners with Unity, at the moment.

Here is a changelog:

- use v2 metacity theming spec
- use less rounded corners to workaround aliasing
- make the left/right/bottom borders slightly darker,
to make the gradient on left/right less evident
- make title frame slightly lighter, to enhance contrast
with respect to darker backgrounds (e.g. other windows)
- larger shadow for focused window

Installation instructions:
just download the file and, with root priviledges, move it (and rename it if necessary) to
/usr/share/themes/Ambiance/metacity-1/metacity-theme-2.xml
Please pay attention to the "2" in the filename!! It must NOT overwrite any original file!

Change theme to something else and then back to Ambiance to see it in action.
To remove: just delete the file. Again, pay attention to remove the file with the "2".