But what are you using to run your reports in a background batch? Do you have a program that kicks them off? If so, in that program you can set the date param value to the current day before generating the report.
Another option would be to set the default date to something outrageous like 01/01/1900. You would then have a formula something like this:
If {?DateParam} = Date(1900, 1, 1) then
CurrentDate
else
{?DateParam}
You then use that formula in your selection criteria instead of the parameter value itself.
-Dell