Reboot to add TempDB files?

No. This code runs without the need for any sort of restart.

ALTER DATABASE [tempdb]
  ADD FILE (NAME = 'testfile', FILENAME = 'D:\Databases\MSSQL10.SQL2K8\MSSQL\DATA\tempdb_extrafile.ndf') 
TO FILEGROUP [PRIMARY];

Edit -> After a little bit of extra background searching, I found this MSDN support article. That one applies to 2000 and gives some extra information on how tempdb files are used.

Matt is correct - you can add files online. I initially thought that this wasn't possible and penned my answer below!

I'm not going to delete the answer, but just wanted to clarify that changes to tempdb, such as moving the files, would require a restart.

Changes to tempDB will take affect when you restart the SQL Server service related to that instance. Whilst a server reboot will achieve this, you can simply restart the SQL Server instance.