Last line meaning that Blitz3D [app] is not going to work at all if you have Vista...
What an odd thing to assume... you quoted this yourself:
The one thing that Microsoft has done with Vista is incorporate a subsystem that will comply with DirectX 9.0 graphics hardware due to the population of users that still own DX9-compliant hardware. This subsystem will be named none other than DirectX 9.0L. So, in short, if you have DirectX 9 hardware, you will be using DirectX 9.0L as your API in Windows Vista.
Do you currently run Windows with DX9? Or are you running DX7 for Blitz3D apps because you think the upgrade will break them?!?
Vista uses "DX9.0L" for DX related backward compatibility. That handles everything DX9 handled - including DX7 compatibility.
www.iexbeta.com/wiki contains a good list of programs that do and don't work on the different releases of Vista to date, including games. There are NO games that don't work on Vista simply because of the version of DX that they use. I play Dungeon Keeper 2 - DX6.
I will say that the DX7 (BMax) and DX9 apps (Sims 2, Black & White 2) I have tested extensively suffer from very noticeable slowdown compared to running on XP. It's not enough to hamper a game, but you can tell it's there. The OGL apps I tested (BMax only) either run at the same speed as OGL on XP, or unnoticeably faster.
Vista will ship in 2007, so mainstream games that require it should start appearing in 2012 or 2013. So much can happen in that kind of time period that we ought not even consider it.
I found that interesting. After such a long feature on DX10 and the marketing hype that came with it prior to Sweeney's interview, he states very simply that Vista and DX10 are probably not going to be worth upgrading to for 6 years. Given that anything could happen in that time, let's cut his estimation by half. 3 years is still a very long time for a 'gaming' OS to stagnate before becoming worthwhile.
If MS had found a way to incorperate DX10 into at least XP, this would be somewhat different... but as it stands DX10 is really the only new thing Vista provides that is actually worth anything... and even then, it's not worth enough to be classed as a must have upgrade for gamers.
In 3 years, I predict DX11 will give us new reasons to reconsider Vista.