Epocrates has just released a free BMI & Obesity MedTool for Palm OS. It has a very quick slider input for height and weight, calculates target weight and Basal Metabolic Rate. It also has the most recent criteria for pharmacotherapy and bariatric surgery.
The tool is sponsored by the American College of Cardiology and was developed by me as a way of calculating BMI in a more rapid way, as well as answering the inevitable "How much should I weigh?" question in an objective manner.
Also, it's nice to have something to look down at while patients stare at you with an incredulous look their face. Feel free to shrug your shoulders and say, "This darn thing says you're obese." I do it all the time.
MedTools are accessed via Epocrates Rx by tapping the "black bag" icon.