how to build my HDTV converter?

I doubt it would be simple to slap together, but I just read here in S.E.D. that off-air digital boxes are "$199"

Yes. Not a simple device at all. However, if one must roll their own (for hobbyist purposes or to bypass the broadcast flag security) here's some info:

You can use a standard VHF/UHF Tuner module. Many are available and have I2C controls. Next you will need an 8VSB demod/decoder. Some years ago, I picked up a reference PC board from Philips. They used an TDA8960. The output of this will be the 'transport' data stream, which will have to be demultiplexed into individual program streams. Pick your favorite FPGA for this. Next, you will have to decode the audio and video MPEG streams. There are numerous options available here (same chipsets used in PC video and audio cards).

What are the ATSC tuner maker? What are the IC suppliers? Any schematic available?

You need one that specifically will work with ATSC (or generally have an IF). Most standard tuners nowadays will output analog A/V only.

The easy way, what the STB manufacturers use, is an all in one decoder CPU (if there are any HD ones), which may directly take the T/S directly. Oh, and you will have to write firmware yourself, which will mean buying an SDK, and licenses for at least MPEG2 and AC3, not to mention the time and cost of creating a PCB for it all (and the cost of getting it wrong the first few goes).

For playing around, how about an ATI HDTV Wonder card? Needs a 1.3 gig processor, 256 meg RAM, 64 meg video card and an antenna. List price $200. Recording software is included. It uses the Philips Maui module. GG

Probably the usual, Sharp, Philips, Thomson, and such.

ST, Broadcom (I know of, there could be cheaper chips you could integrate into digital TV receiver.)

The datasheets/devkits are a good start.

Does anyone has experiences of these chip sets out there to share with us? Thank you.

available