There are a few other things worth mentioning if you aren't aware.
1. Make sure you are watching a HD channel! NBC is likely on something like channel 4. The HDTV version of NBC could be on channel 304 or 504 or 704.
2. As mentioned above, make sure your cable box is set to output at HD resolutions such as 1080i.
3. Make sure you are watching a HD television show. Just because you are on a HD channel does not mean that you are necessarily watching a show which was filmed and being broadcast in HD. Most prime time shows on major networks are in HD.
4. Double check that you are on the HDMI input for your TV! Some people hook up HDMI, but then leave composite or even worse, their F-connector cable connection attached to the TV, then never switch to the proper input. If you don't have your TV on the HDMI input, you aren't likely to be watching HDTV.