Order posts by most comment date

I have dredged up every corner of internet to find a straight answer to this issue, but I have had no luck. I created a wordpress blog for a Business Association I run and I need to order my posts by the most recent comment. This blog is being used as a sort of messageboard and I want active conversations to stay at the top. Is there an orderby command I am missing here or is this not possible?

I sup[pose there might be some sort of SQL Query to accomplish this, but that kind of language is a little out of my league. Does anyone have an clue on where to look for a solution?

If you are wondering, there reason I used the WordPress blog itself as a information exchange system instead of an actual messageboard of some variation of WordPress MU is that most of the people in this business association are computer illiterate. The setup I created is actually surprisingly user friendly, but as you can see, it has its limitations.

I appreciate any advice.