Make certain that you have all Office & OS X updates applied as well as any
available firmware updates for whatever model of Mac you have. If you're
using a 3rd party keyboard check the mfr's site for a driver update.
Also, repair disk permissions & restart your Mac.
There is no known conflict of that nature - I run Excel 2008 in SL on a MBP
with no lack of response from the arrow keys, so the problem must be local
to your installation. If the suggestions above do resolve it the next most
likely cause would be any helper/haxie apps you may have installed. You can
test by logging in while holding the Shift key & launch Excel while no other
programs are running. If all works as it should it's a sure indication that
some other program is interfering with keyboard operation... It then becomes
a process of elimination to determine which one is the culprit.
HTH |:>)
Bob Jones
[MVP] Office:Mac