SSRS date default with formula disables parameter
After playing some more, I realized that the date controls became enabled when I picked a value from a preceding dropdown parameter that did not have a default. Apparently, controls after non-default parameters are disabled until you pick something, so order matters.
From an MDSN article:
"parameter order is important when you want to show users the default value for one parameter before they choose values for other parameters"
http://msdn.microsoft.com/en-us/library/cc281392.aspx