Absolutely! Playing the uke combined with good diet and other forms of excersize can be part of a valid wight loss program.
Playing the uke burns a significant amount of calories compared to static activities such as reading, or watching T.V. There are things you can do to increase the number of calories burned during practice, or playing such as- standing and moving around (dancing, swaying, tapping your foot) all of these motions add to the calories you are burning.
For example, if a 200 pound person plays the uke for 1 hour (seated), he or she will burn 288 calories. If the same person plays fo 1 hour standing they burn 395 calories. Add a little fancy foot-work, and you're up to 564.
1 hour of sex, by the way burns between 380 and 500 calories depending on how um... rigorous you are in the sack.
By the way, the more weight you lose the more calories you will burn per minute of excersize (rational to your body weight), that is why its easier to stay in shape than to get that way... and also why our friends in the previous post will probably have to do a little more than strum...