How do you cope with Database Server Inventory Management Is it common for SQL Server DBAs to have to struggle with Server Inventory Management due to multiple parties / departments bringing online their own instances of SQL Server?

If so have you been able to successfully add controls to prevent this problem?
Also how do you monitor your environment on a regular basis to troll for unmanaged instances so that you can proactively prevent embarrassing situations like databases without backups?
I don't have direct experience with this task but know how folks get around this problem. :-) 

You setup some monitoring program to look out for all SQL Server instances on the network and anytime something comes up that is NOT in your radar, you can catch them right away. There are lot of options and here are couple of them. I am pretty there might be others or many variants of these two.

http://www.straightpathsql.com/archives/2009/02/what-sql-instances-are-installed-on-my-network/

http://andrekamman.com/find-sql-servers-on-your-network-with-powershell

That said, I have to tell you that if someone brings an instance without notifying the DBA or production support team then this should be handled politically. Get your boss (in confidence) to talk to their boss and avoid future recurrence of the problem. GL.