The other day I was setting up trace flags when chasing a deadlock issue. The way it works is that after you run DBCC TRACEON, SQL Server outputs detailed log to the SQL Server event log. But for some reason this trace was just not showing. The only thing I could see in the event log was a message that DBCC TRACEON was executed, but nothing after that. I used Google and read some articles but still didn't find what the problem was. I went back to BOL and I finally noticed the reason why:

"In SQL Server 2000, a simple DBCC TRACEON (1204) is enough to enable deadlock reporting to the error log. In SQL Server 2005, you must enable the flag globally because the session-level flag is not visible to the deadlock monitor thread."

So as it turns out, it's a change in behavior from the previous version. Here is how you enable the trace globally:

DBCC TRACEON (1204, -1);