There is no native functionality in Salesforce.com to send an e-mail reminder when a task is due. However check out this useful discussion thread on LinkedIn to find out how others are trying to achieve the same.

http://www.linkedin.com/groupItem?view=&srchtype=discussedNews&gid=66190&item=84398212&type=member&trk=eml-anet_dig-b_pd-ttl-cn&ut=3WqS8A9VgVel41