At what stage do you start bringing in power users into the day to day administration of your community, & how do you decide what tasks to delegate?

The first stage would be to identify those power users -- or, rather, power contributors -- and do something to reward them for their efforts. It doesn't need to be anything monetary or overt. In most cases, simple recognition would do -- namely in the form of promotion.

Promote power contributors (which can be defined by either the frequency of their posts or the quality/relevancy/community ratings of their posts...or a mix of both) by giving their posts preferred positions on relevant community pages. We've tried "unexpected rewards" in the form of discounts on training or passes to real-world events. And we're toying with taking a page from #foursquare's book and attaching special "Top Expert" badges to their profiles.

See how they perform with these rewards before handing over the reins of your community.