Groupset compatibility

Both of my rear wheels have 10 speed cassettes and they are both 8/9 speed hubs - no problems. One is a shimano cassette the other sram. Incidentally this is all with a '7 speed' derailleur.

I think that there is a different spline on Dura-Ace 10 speeds. For this reason I think that there will be 10 speed cassettes out there that won't fit your hub, but equally there will be cassettes available that will

Agree with bigmel on 'upgrading' to 10 speed. If you are upgrading just for an extra cog I think you are on the wrong path. If you are upgrading to better performing kit I can see why you might want to go to 10 speed at the same time though. In my view, however, it's the shifters that really make a difference.

Very surprised at what you were told. A hub body that takes a Shimano 9-speed cassette will actually take an 8 or 9 or 10 speed cassette as mentioned here.
Not sure an extra cog is an "upgrade" though - I use 9 speed on most of my bikes because the chain sideplates are thicker and last longer - 10speed chains are narrower and definitely don't last me as long.

I fitted a 10 speed Shimano cassette on a bike that originally had a 9 speed cassette on it's freehub last year. It worked a treat with a spacer and a bit of rear mech fettling. I also managed to get a SRAM 10 speed cassette to work.

If you're sticking with Shimano Hyperglide, you can fit a 10 speed cassette on a 8 or 9 speed hub using a 1mm spacer.

More info from Sheldon Brown ... http://sheldonbrown.com/k7.html

You can fit other manufacturer's cassettes on too ... http://sheldonbrown.com/k10.shtml