How to make a field required based on created date?

Hi,

On the service request object, i have added a field that needs to be required however since we have been using the crm for a year now, there will be previous records where the field has not been filled in.

I want to make a field validation that if the created date is equal to or past todays date time (05/01/2012 12:30) then the field is required whereas if the created date is before todays date the field is not required.

Does anyone have any ideas how to do it?