I'm looking into buying an Avermedia A180 for HDTV capture of over-the-air broadcasts of HD content (I live very close to the broadcast towers). Is it possible for me to use my current setup of a PVR-500 to capture SDTV along with the HDTV card? I imagine that switching tuners would mean that they would recieve different channel listings.

Summary: Has anyone had sucess with SDTV cable and HDTV antenna broadcasts in the same box?

Yes, it's possible. Why wouldn't it be? It's just one of the many possible multi-source configurations. Not just each card in your box, but each input on each card can have a different source...

This is exactly how I have my mythbox configured right now, and it works quite nicely (PVR-150, Air2PC-HD5000). If you are using zap2it, make sure you create a second profile, one for your cable stations and one for the OTA broadcast stations. Then, when you setup your cards (in mythtv-setup) make sure you select the appropriate profile for each card.

So when I switch to the HD tuner, it will only change channels to the ones I have specified on zap2it? I imagine that mythweb will just show the antenna channels along with the cable ones too.

Not exactly. The channel needs to be setup as a valid entry in myth's database. Just having the channel selected in your zap2it account doesn't mean you'll be able to switch to it in myth. Just make sure that the channels you've set up (during your channel scan or those you manually enter) match the channels you include in the zap2it profile.

You don't want to have a bunch of extra channels that don't give you a solid signal. This is especially true for DVB cards. (Switching to a "bad" channel can cause myth to lock up) Just because a channel may be available according to zap2it, you may not, in reality, actually be able to receive that channel at a high enough signal strength.

As for mythweb, the OTA channels will appear right along with the cable channels in the program listing. I suggest using a different channel format (like 5_1 or 5-1, etc.) for OTA channels so you can easily tell the difference between the cable channel and the OTA channel.