We are using a trac post-commit hook script that will search your commit messages for text in the form of:

   command #1
   command #1, #2
   command #1 & #2
   command #1 and #2

You can have more then one command in a message. The following commands are supported. There is more then one spelling for each command, to make this as user-friendly as possible.

closes, fixes

The specified issue numbers are closed with the contents of this
commit message being added to it.

references, refs, addresses, re

The specified issue numbers are left in their current status, but
the contents of this commit message are added to their notes.

A fairly complicated example of what you can do is with a commit message
of:
Changed blah and foo to do this or that. Fixes #10 and #12, and refs #12.
This will close #10 and #12, and add a note to #12.
The above was taken from the svn-trac-hook python script.