http://www.sqlskills.com/blogs/paul/misconceptions-around-tf-1118/

[Edit 2012:] 4a) What is Paul’s recommendation for using trace flag 1118? Everyone should turn it on, on all instances of SQL Server, from SQL Server 2000 onwards. There’s no down-side to having it turned on.

http://www.sqlskills.com/blogs/paul/a-sql-server-dba-myth-a-day-1230-tempdb-should-always-have-one-data-file-per-processor-core/

But there’s now even better guidance, and what I also recommend. At PASS in 2011, my good friend Bob Ward, who’s the top guy in SQL Product Support, espoused a new formula: if you have less than 8 cores, use #files = #cores. If you have more than 8 cores, use 8 files and if you’re seeing in-memory contention, add 4 more files at a time.

http://technet.microsoft.com/library/Cc966545

Use TF-1118. Under this trace flag SQL Server allocates full extents to each tempdb object, thereby eliminating the contention on SGAM page. This is done at the expense of some waste of disk space in tempdb. This trace flag has been available since SQL Server 2000. With improvements in tempdb object caching in SQL Server 2005, there should be significantly less contention in allocation structures. If you see contention in SGAM pages, you may want to use this trace flag. Cached tempdb objects may not always be available. For example, cached tempdb objects are destroyed when the query plan with which they are associated is recompiled or removed from the procedure cache.