I agree with SQLSister 100%. I resigned 2 weeks ago and I did it professionally. Even took time to send an email to all of my co-workers explaining to them where all of the documentation was and how to do the tasks I did.
In my situation, the company I worked for was a train wreck :). In addition to that, they wanted me to start doing web design (I'm a color-blind C++ programmer), and that's when I started looking for a new job. My resignation was a total of 5 or 6 lines. Basically, I just said that I felt that I needed to move on to a new challenge, it's not their fault...it's mine...etc (wow..that sounds like a break up letter).
Even after you're polite, be prepared for an attempted backlash. Just take the high road and don't respond to any negative statements. Remember...you're the one who is going on to a better place :). In my situation, my wife worked at the same company (she is a web designer and enjoys it). The owner had her network access taken away as well and it looked like he was firing her, but he eventually realized that she was the best designer they had and gave her access back.
Long story summarized...put on some thick skin, and be nice. As far as what you tell them in your resignation letter, be honest; but you don't have to divulge any further information than what you want.
One other thing I just thought of. Read over your employee agreement/handbook to make sure there's not going to be any issues with non-compete/invention assignment/whatever. It would suck to find out you legally can't take the new job after resigning and they won't take you back :).
im in ur stakz, overflowin ur heapz!