I call BS. The farcry demo, which is DX9, runs well on my 9800 Pro(although that hardly falls in the "mainstream" level..). Even though the AA doesnt really work yet. As well, Half-life2 would run as low as DX 6, if it had to. So it cant be a compatibility problem.
The NV40 design(which is what the Nvidia FX cards are based on) have horrible DX9 support; which is why it needs to run in mixed mode(DX8.1 and DX9 graphics) to get tolerable frame rates in Half-life2.
Then again, maybe today's generation of cards are slow. They are releasing most of the dx9 games when r420(the next generation of Ati's cards) comes out(dunno if the next line of Nvidia cards come out then).