I have the same issue. The DropDownList won't submit an empty value. This seems to be a common use to have an option label which has an empty value. Is there a way to make this work properly?
Please check out the following fiddle which illustrates the issue:
http://jsfiddle.net/pL4JA/1/
When "Select a value" is selected and you click the button -- the value of the dropdown should be empty not "Select a value".