Metacity hands focus to new windows. This regularly means that hitting return in a terminal window as evolution fetches new mail cancels the fetch. It means new irc windows steal focus from web browsers and people type credit card data onto irc. Its unpleasant from a usability point of view as well. New windows should not steal focus unless they are stealing it from another window of the same application (eg dialog boxes) or specifically request to do so.
If you want the very long version of how this specific problem is being addressed: http://cvs.gnome.org/viewcvs/metacity/doc/how-to-get-focus-right.txt?rev=1.3 One of the bugs mentioned there is still open to remerge the focus stealing prevention into GNOME 2.10