I'm trying to figure out my cabling and product needs for an upcoming
upgrade. I'd like to get a HDTV-capable 16:9 tube tv soon (I'll wait
until prices drop on LCDs in the next couple years), and I've
currently got a triple LNB dish running two cables to a DirecTiVo box
that is working great, wired via S-video and digital audio to my home
theater system. I have a clear sky near the dish and it seems to get
the 110 satellite fine on the test channel. What I'm wondering about
is once I've got the new TV in place, and if I pick up a HDTV OTA box
to decode the signals, will I need to run a third or fourth cable down
from the satellite dish to my TV setup for HD content? Will I need to
incorporate a switch to view either the satellite TiVo content or the
HDTV content separately? Last question: is OTA HDTV decoded from
settop boxes done with composite video cables or DVI only? If my home
theater receiver/amp doesn't have composite, will I need to upgrade
that too?

I am the Chief Engineer for a post-production company and am also a
broadcast production engineering consultant.

To receive free OTA HDTV from your local broadcasters, you will need
to run a cable from a standard rooftop antenna to your set top OTA
decoder.  The satellite dish will not act as an antenna for OTA, only
for pulling in the DirecTV content (which can be HD if you pay for it,
and you have a HD DirecTV receiver).  The size of antenna needed
depends on your geographic location.  Small antennas may be attached
to your satellite dish.

To view true HDTV from your OTA receiver, you will run three component
cables from receiver to TV(three standard RCA cables).  DVI is not
needed.  Good luck!

-Erik

So if I wanted to see OTA content and DirecTV HD content, I would need
both an additional cable coming down from my dish, plus a new rooftop
antenna to get the OTA signal.

Now, does this mean I need both a OTA decoding box AND a DirecTV HD
decoder as well? Or does one box do both?

Then I'm guessing I'll need some switchbox to jump between DirecTV
regular signal, OTA HD, and DirecTV HD signals. I'll probably wait for
DirecTiVo with HDTV capabilities if it's going to be this complicated.

Yes, you are mostly correct.  SOME DirectTV dishes have a built in OTA
antenna, so there is a slight chance you allready have one, otherwise
you will need a rooftop antenna with a cable to the receiver (I think
some even have provisions to send the OTA signal down the smae cable).
 There are several companies that make HD OTA tuners combined with
DirecTV, so you will not need an additional box (they can also tune
the direcTV HD channels) Sony, RCA, and Samsung come to mind.  I'm not
sure, but there may be one that has a tivo too!

You will not need a switchbox as the single DirecTV tuner will do it
all,.. though it will have to be one made for this.