I too am in Elkridge/Howard County, but have never seen TNT HD or Discovery HD in the clear. I only get a PBS, CBS, ABC, NBC and the Comcast Sports Net in HD (I get more digital channels, but they are SD, not HD). Recently, I rescanned, and now the channel assignments are somewhat garbled (PBS is showing up at 13-2 or 13-1, can't remember which one... but it definitely shouldn't be, and now CBS HD is missing. And, I started miraculously getting Fox HD at 45-2 (I only got Fox HD via OTA before)).
Anyways, other than ComcastSportsNet HD, I've never gotten anything other than the local OTAs (and even then, FOX HD hasn't typically been on my Comcast feed as unencrypted).
I have a Vizio VX37L set with NTSC/ ATSC/ QAM tuner built-in.