I think 100Hz is a vital thing when purchasing LCD tv's as their motion on 50Hz is not as good as a Plasma screen for example, a 50Hz screen will still give you a good picture but soon as something with a big sudden movements like a sports broadcast wont look as sharp as it can be and leave a slight blur on the screen, even normal movements look clean and sharp on a 100Hz screen.
So in regards to taking the tv back, i cant really answer, if you are happy with the picture on the U10 then keep a hold of it as the U10 series is a basic entry level Panasonic and affordable. If you decide to take the tv back, then you will have to upgrade to the "G10" series which is the cheapest Panasonic 37" LCD with 100Hz but you will have to pay a bigger difference compared to the U10.
Regards> M