It has been recommended for SQL 2000 and 2005 to enable trace flag 1118 to disable mixed extent allocation.

Recently a hotfix came out for SQL Server 2005 SP2 that resolved a potential contention issue with the flag itself: http://support.microsoft.com/kb/936185

Linchi has not been able to duplicate the issue, but the hotfix is there if needed now.  Hopefully the hotfix will make it into an SP3 sometime so setting this flag can be done with more confidence that it's not going to make contention worse.