After upgrading to Ubuntu Lucid, I didn't like some aspects of the new theme (Ambiance). Here the fixes for the two of those things.
A panel wider than 24px looks ugly
Another thing is, that the background image for the panels doesn't scale (See Launchpad Bug 532309). To fix this, edit the file /usr/share/themes/Ambiance/gtk-2.0/gtkrc and comment out the line bg_pixmap[NORMAL] = "panel_bg.png".
#bg_pixmap[NORMAL] = "panel_bg.png"
As an alternative, if your panel is exactly 30px wide, you can also replace panel_bg.png with panel_bg_30.png.
bg_pixmap[NORMAL] = "panel_bg_30.png"
After that, log out and then log in again, and the panels should be fixed.