By accident I found out that when I login with the default English language, the icon for mailboxes in Mail looks much nicer than in foreign languages. If you want to have a look or to change it, go to Mail (Control-click and select Show Package Contents) -> Contents -> Resources -> mailbox.tiff.
There is another icon there only for the English one -- a file named mailbox_en.tiff. To change your view, rename the original mailbox.tiff to mailbox.tiff.old, make a copy of mailbox_en.tiff and rename it to mailbox.tiff. Restart Mail and then you will have the original amerikan mailbox.
[robg adds: I only run US English, but I have the mailbox.tiff icon in my drawer view; copying and renaming as above didn't have any effect on the displayed icon for me.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040611101308310