That is true. ALSA intel8x0 driver does NOT support hardware mixing in MCP-T.
Nvidia's OSS 1.0-310 driver uses MCP-T hardware mixing. That is why I bought (and like) nForce2 mainboard - great MCP-T hardware mixed sound for free (because it is integrated). With Nvidia's OSS 1.0-310 you get also hardware accelerated OpenAL library. Nvidia is the only company which provides hardware accelerated OpenAL in Linux.
I'm very unhappy knowing that nForce2 was the last chip with APU.
I'm also waiting for ALSA driver made by Nvidia.