I recommend that you keep the written resignation short and business-like. No need to say why or where you are going after this. In any discussions, you can elaborate, but I would not tell them that you are going to work for the competition. Just say that you have another opportunity. And smile and thank them!
Please accept my resignation as of today, effective November 15, 2009.
I appreciate the exceptional opportunity you have provided for me here. You and the company have been more than fair with me, and I genuinely value all your support.
I wish HRDT continued success, and I want to thank you for allowing me to be a part of your team. Please feel free to contact me at any time if I can be of further assistance in helping with a smooth transition.