Is there any reason to wait and buy an HDTV until a 2008 (post 2/18/08), when (I assume) manufacturers cease including an NTSC tuner in their sets?

If your wait is based on the assumption that the NTSC tuner will be discontinued, I believe you may be incorrect.

The NTSC tuner will still be necessary for your television to tune signals such as those from cable television providers.

The ATSC tuners will, of course, be required by law in order to receive digital OTA (over the air) from television broadcasters.

I think we can expect both NTSC and ATSC tuners in our televisions for some time to come.