The only game known to even attempt to use DX10 is Crysis as Nextman916 stated. Most likely it will use a DX10 compatibility mode. There is zero confirmation that it will fully utilize DX10 and not just a couple of features. DX10 isn't even, at least to my knowledge, finalized yet. Many many games industry folks have stated it takes roughly 3 years to make a decent game.
Even if someone started a game using DX10 on non final API's on Jan 1, 2006 it would take them at least two years to make a game and likely 3 years for a decent one. Which would put it at 2008-2009 timeframe. Basically you're likely to not see a game truly utilize DX10 until mid 2008. We saw the same thing with DX7, DX8, and DX9 where it took a while before developers truly utilized the new version of DirectX in making their games.
For Crysis to use DX10 it will most likely only use a few features of DX10 that won't enhance the game by much. Think of it as the difference between the early DX9 games and DX8. Some games ran in both modes depending on your hardware. When screencaps were taken between both games the graphical difference (aside from water effects) was remarkably small. Which, I'm betting, is the case with Crytek's new game. DX9 had a pretty big efficiency advantage over DX8 but I'm not so sure that DX10 will add as much as far as efficiency goes but be more along the lines of adding effects and increasing the versatililty of the API's.
In late 2002 and early 2003, we were only just starting to see more than a handful of games utilizing DX8 and DX9 was released in Dec 2002. It wasn't until 2004 that we really saw any signs of life from DX9. Nearly two years after the release of DX9. Why shouldn't DX10 be any different than DX8 and DX9?
Not only does the timing of things make it highly highly unlikely for a game that truly utilizes DX10 to come out, the economics of it don't make sense either. Vista will have an extremely small installed user base compared to WinXP and it would be financial suicide to make a game that is optimized for DX10 and not one optimized for DX9 at this time that will be released in 2006 or well into 2007.
I think if you want a good look at what DX10 will bring to the table you should look at Xbox360 games. The GPU was developed by ATI and contains unified shaders. The graphical effects available on 360 are what I'm expecting on the new R600 and G80 GPU's. They're good, especially for a console, but hardly heads and tales above games we've seen before.
Either way, the demo reel of Crysis did not reveal anything that wasn't possible in DX9. If you are talking about a different game that's going to be out sooner, please state it.
I stand by my original statement, there is no sense waiting for DX10 in 2006. I think mid 2007 is when one should seriously start looking towards DX10 parts. I think my reasoning is pretty sound as to why one shouldn't wait for DX10 and not based on ignorance as you claim. And if you bothered to do a search, this question has been asked more than once. Offhand, I remember at least 3 threads on it in the last couple of months alone.
So if it's time for you to upgrade now then do so now. As I will as soon as I find out which comes out on top between the X1900 and 7900GTX. If your normal upgrade cycle happens to coincide with the release of DX10 compliant parts, then good for you. But I wouldn't hold off on upgrading just to wait for DX10 as no games will make use of it for close to 2+ years.