What happens when a temporary pilot certificate expires?
According to this AOPA article, you can request an extension online through the FAA's Airmen On-Line Services.

You also can look up your current status here: 

http://www.faa.gov/licenses_certificates/airmen_certification/new_verification_info/

In my case, this was updated about a week before my card arrived in the mail.

You can also see what date's submissions they have processed here: 

http://www.faa.gov/licenses_certificates/airmen_certification