I'll add my two cents. My hubby and I bought a Precor treadmill a year ago as our dual Christmas present, and so far, so good. I believe we spent somewhere between $1500 - $1800, I think we could have gotten a cheaper model, but the more expensive models had programs which we thought we would use. Actually, I found that I usually just get on it and go, and my hubby does more of the programs, but it's nice to have the options.
My main recommendation is to make sure you are getting a sturdy machine. I would definitely suggest going to a store and trying out different models. I used to run pretty heavy and like to run fast intervals, so at the stores I kicked that baby up as fast as I could handle, and put the incline really high to make sure it wasn't shaky. The salesmen didn't think anything of it.
Also, even though I know many people who bought online and and at general sporting stores and have been happy, the specialty stores seem to offer much better service/warranties. You want to know that if something breaks down, it will be taken care of and you won't be paying $$$$ for repairs. Also, the specialty stores are more willing to negotiate price with you. They want your business, and pretty much all of those sticker prices are starting points.
Everyone seems to have made some great suggestions and I know we really like our Precor, but in the end it's what works for you. I think a treadmill is a great investment and you will get a lot of use out of it.