<p>I know that you can do this in SQL Server 2005, but I'm at a loss for 2000.</p>

From all the research I've done on this in the past, I unfortunately have to say no. SQL Server 2000 simply does not store this information, and I've never seen any solution for retrieving it.
There are a few alternative methods, but they all involve user intervention. Besides keeping stored procedure scripts in a source control system, I think the next best approach is to use comments inside the stored procedure. Not ideal, but it's better than nothing if you want to track what gets updated.