Known issue, unfortunately. Bethesda moved the hotkeys/favorites from their traditional location (the top number row, 1-9) to the numpad. Because hotkeys/favorite are traditionally hardcoded, that meant the numpad was completely locked down. Why? Who knows, but I'd like to smack whoever made that call around with a trout, because it made the game practically unplayable out of the box for me as a leftie.

There's various workarounds for this, but the one I've used (and had no issues with) has been http://skyrim.nexusmods.com/downloads/file.php?id=88 Just drop the "numpad rebindable" controlmap.txt in the appropriate folder and you'll be good to go--you can now rebind keys to the numpad again.
FYI, the on-screen tooltips will still show the default keys for actions no matter what you do, since they've been hardcoded.

You may also experience some weirdness and brokenness if you rebind some options to some of the original defaults--the game gets cranky if you make "T" (default for wait/charge items) serve another function, for instance. Rebinding "R" was fixed in the 1.3 patch, though. (It now opens the spell menu in my game, and I've yet to have the game explode over it.)