can i use a 9 sp cassette with 10 speed shifters? would there be any problem?
The pull ratio is going to be different, which will raise havoc with your shifting and adjustment. There is one trick that kind of works:
http://sheldonbrown.com/derailer-adjustment.html#alternate
If you use the Alternate Cable Routing B method you can get a 10 speed shifter to work OK with a 9 speed cassette. I would suggest, though, that you buy a new 10 speed cassette and RD.
Just a question (or clarification); aren’t RD’s dumb?? (not Race Directors 
). The shifting distance is controlled by the shifters, so wouldn’t a 9 speed RD work with a 10 speed set up, and vice versa? Just asking, but thought if he just has to buy a 10sp cassette then that’s a lot cheaper/easier?
Just buy a 10 speed cassette.
Probably have to buy a chain too
jaretj
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Just a question (or clarification); aren’t RD’s dumb?? (not Race Directors ). The shifting distance is controlled by the shifters, so wouldn’t a 9 speed RD work with a 10 speed set up, and vice versa? Just asking, but thought if he just has to buy a 10sp cassette then that’s a lot cheaper/easier?
Head slap 
You are correct.
you also run the risk of shifting the chain in to you wheel, or off the bottom end, neither of which are wonderful prospects
.
When mixing 9 and 10 speed equipment, the cassette, chains and shifters have to be the same (9 or 10 speed)
Der and crank are usually interchangable between 9 and 10 speed.
I *think *the oldest Shimano 10 speed bar end shifters have the little ring thing on the side that enables them to be switched over to friction mode - in that case you can run whatever cassette you want, you just won´t have any indexing.
I am fairly sure that the newest 10 speed BES from Shimano do not have this feature, and therefore are only compatible with a 10 speed drivetrain. Fairly stupid of Shimano to exclude any compatibilty with other drivetrains for the newest shifters, but there you go.
As far as I am aware any Shimano RD will work on any setup (6,7, 8, 9 or 10 speed), with some exceptions (early 7 and 8 speed Dura Ace used a different pull ratio), so in theory your crappy old $10 SIS Shimano RD from a Wal-Mart mountain bike will work on a 7900 Dura Ace drivetrain.
Regarding getting round the different ´pull´ratios, Sheldon has a fix that involves an alternative cable routing on the RD. I couldn´t get a set of 8 speed GripShifters to work on my 105 drivetrain until I rerouted the cable on the RD, bingo, now it works perfectly. A previous poster mentioned this fix too:
He also says:
***This alternate cable routing will also let you use a 10-speed shifter with a 9-speed cassette. This provides a convenient upgrade path. ***
Standard cable routing:
Alternative (90 deg bend in the cable)
Hope this helps.