Depends on how you have things set up. I once worked at a company with SVN/Trac integration. If you put "Refs #27" in a commit message, it would actually add a link to the commit as a comment on the ticket. If you put "Fixes #27", it would do that and close the ticket. Our system was also set up so you had to ref or fix a valid ticket in order to commit.

I miss having a system like that...