In DX7, it would be a waste of time to code because you have to use the card's drivers specifically for T&L... With DirectX API (instead of GLIDE, RRedline, etc- various APIs) you can code once for pretty much every card and tweak just a little.
If you code directly to their APIs (RRedline for Rendition Chipsets, GLIDE for 3Dfx Chipsets, etc) you have to have several sets of code to make a game work for different cards.
With DX it's much more easier, although you don't quite get the performance you would if you used the card's manufacturer's API, you still get stable code and very few graphic anomolies with much less work.
In other words, when DX8 is fully incorporated and working, T&L should be supported because it is easily programmed to from within the DX8 API.
-SW