SQL Server 2008 R2 SP1 - Tempdb "Available free space" showing negative value
Open Query Analyzer
Run DBCC UPDATEUSAGE('tempdb')
After it is done, check temdb value and it should show the true values.

Also, this might help you.

Also, check if there are any open transactions using dbcc opentran or sp_whoisactive (Adam Mechanic's SP)

This was a bug in SQL Server 2008 SP1 described here.

Out of curiosity, is it causing any performance problems ?