With the Air2PC card I have, it was similar to the above, ie it scanned the whole spectrum and came up with a list of channels, however if I hadn't been aimed right, it wouldn't have come up with anything.
When I use TSREADER, it comes up with a window where I enter the physical channel (38, 44, and 46 in my case) and TSREADER automatically fills in the frequency and pops up a display of whatever channels are there, and I can choose which one to play (with VLC).
I just got a NEW ATSC gadget today, since I'm in the market for an
HDTV, and wasn't sure if I was going to get a monitor or TV. What I got is a set top box type tuner, and it worked like Corey said too, ie I just told it to scan the whole band, and it came up with all my channels. A lot more convenient that using the computer, however the computer card gives me the option of high quality saving to disk.