I’ve been testing Ubuntu Jaunty for a few weeks now and after using it for a day or two, my memory filled up. Recently I found out that this was caused by Compiz. Apparently if you use UXA with the new Intel drivers, Compiz will start to leak memory. After only 8 hours of usage, Compiz was using more than 1.5GB of memory! The solution? Disable Compiz, because UXA is a big performance improvement for me.
Update: Apparently this bug is fixed (#328232).