11 speed - 10 speed compatibility.

Yes, thought this might be the case. If I get the 11 speed hub will I be able to put the 10 speed casette on it (with some sort of spacer perhaps). The componants on my Tri bike are all Dura Ace so really can't justify spending that amount of money to switch over to 11 speed.

You can use a 10 speed cassette on an an 11 speed free hub but not the the other way around. As long as you have a 10 speed cassette, you can use that on your 10speed bike. And of course when you want to use this wheel for your 11 speed bike, you need to change the cassette over to your 11 speed cassette.

If you are asking what it would take to switch it all over to 11 speed; shifters, rear derailleur, chain, cassette. Although you can get away with using your 10 speed front derailleur, I would recommend changing that as well.

Well, not exactly.

An interesting hack will allow you to put a modified 11 speed cassette on your 10 speed wheel. See:

http://darkspeedworks.com/blog.htm

You need to be a good bike mechanic and well versed in deraillieur adjustment - the rear mech will need to be retuned, including both the upper and lower stops. But its possible, and it works. Do not recommend trying if you are not a semi-expert mechanic. The reverse - using a 10 speed on an 11 speed hub just requires a spacer placement and retuning.

Hope that helps.