Nice answer! You may not love but it is certainly a challenge!

The 'sa' account as you have mentioned becomes a risk if someone can get hold of the password and that is simply a risk that I want to mitigate as much as possible. The fact is is that if your instance uses Integrated Security then even if the 'sa' account is unlocked, it can't be used. One of the reasons I try and separate those applications that require SQL Server authentication and those that don't. If the 'sa' account on one mixed-mode instance is compromised then it is possible to compromise other instances with a couple of nasty tricks. The fact here is that if someone REALLY wants to hurt you, they are going to find a way regardless. The best we can do is try and mititgate the risks as far as possible. Additionally, our 'sa' accounts (regardless of the authentication mode) have differing and complex passwords.

Your point about the Resource Governor is a valid one and, much as I hate to admit it, a subject that I am far from a master of. To be honest though, although I have received a fair amount of critism (constructive though!) about how I manage memory on my servers, I am happy with the outcome. As I have mentioned several times, teh ability for me to be able to in effect, sandbox activity to within a single instance, has saved me on a couple of occasions. When I consider the amount of time spent setting up, configuring and monitoring this activity in the fist place, I regard it as a good investment. Not only was disruption confined to a single instance but work happening on other instances carried on regardless!

Your point about instance-based software licensing. This isn't one that applies to us as we don't have software that falls into this catagory.

I liked your last statement:

As you may know, the general recommendation is to consolidate instances to remove barriers to full utilization because it allows SQL Server to achieve a more complete use of the available server resources.

Ironically, removing the barriers to provide me with full utilisation offers up (in my situation) the problem of database activity starving other databases of memory resources. This is a situation that could probably be resolved using the Resource Governor but as mentioned, my knowledge in that area is still too thin to provide a solid platform of conformity across the instance. I will look at it but I am loath to fix a running system!

Thanks for your comments!