I've been thinking about this for a while now. It would appear that for some this may be the best way to go.

Best of the Web is doing just this. Before and after the recent PR update. They kept their PR and not sure but think they may have had an increase.

Ordinary basic directories may not stand a chance on this though because of the Adsense cost. It may outweigh the income to the directory.

Any directory wishing to run adsense would be wise to create a keyword targetted landing page to ensure cost per click is kept to a minimum.