Sparse files are not used for TempDB. They're used only for database snapshots. See Books Online (SQL help file) under database snapshots for details.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability
We walk in the dark places no others will enter
We stand on the bridge and no one may pass