something new for you

you can use -moz-border-radius for all four corners
or
-moz-border-radius-topleft, -moz-border-radius-topright, -moz-border-radius-bottomleft, -moz-border-radius-bottomright for each single corner.

i would think of 3 things which would look very cool:
1. have 3 corners rounded, one not
2. have 2 corners rounded, best on opposed sides
3. having different radiuses (<- w00t? ^^) for each/some corners

(point 2 is my personal favorite ^^)

haven't tried this... but i hope that this works... so if feel free to test

for some previews and/or inspiration:
http://www.the-art-of-web.com/css/border-radius/

edit: take a look at the bottom of the page where the "new Firefox short-hand properties" are mentioned. maybe they'll work on the n900 too. (: