First of all, if you purchase an HD TV, it's very important to note whether or not the TV has an
integrated HD Tuner. If it does not, you will have to spend more money to receive even local off-air HD broadcasts. Integrated HD tuners are best.
For off-air HD broadcasts, you do not have to subscribe to anything. These come in through antenna just like regular local broadcasts.
Satellite TV providers do charge additional fees to subscribe to their HD programming (this means in addition to the monthly fee you pay for the actual HD programming). For instance, DISH Network will charge you a monthly fee for your standard definition programming, and another ~$20 for your high-definition programming (about 40 channels), plus other hidden fees (like DVR fees if your receiver is a DVR, etc).
If you choose not to pay for a monthly HD service such as HD satellite or cable programming, you are not wasting money on that fancy TV! That TV can (assuming it has an integrated HD tuner or separate tuner connected) receive local HD broadcasts as previously mentioned. However, you also have the ability to now purchase high-definition video players (e.g. HD DVD, or BluRay) and watch movies with a quality like never before (better than theater quality in most cases!). On top of all that, if you or other family members are into video games, most new gaming consoles (Wii, Xbox 360, Playstation 3) support HD.
One more piece of advice I would like to offer if you're considering making the leap to high-definition: don't cheap-out! Accept nothing less than 1080i, you will thank yourself for it later, trust me!
So I guess that means the next question is, "Which is better, BluRay or HD DVD?"
