I just received some fascinating info from a contact in Yen Sun Technology, better known as Y.S.Tech who you should instantly recognise as manufacturing a large proportion of the fans used with today's PC CPU coolers.
Basically, what they've done is made a fan that is driven, not by a central motor as we're used to, but by an electric motor around the OUTSIDE of the fan.. using the tips of the fan blades. This of course means the normal "dead spot" in the middle of the fan and hence in the middle of the heatsink - where it needs airflow the most - is eliminated.
Here's a scanned brochure with an explanation of the technology - be warned, it's a large image.

Here's what I assume is the other side of the brochure:

Here's some pics of the unit in a typical application:


I've requested more info about CFM ratings and noise levels etc.. of course, I've also requested a review sample and I hope to share the results with you soon! Update: More info on this page on Y.S. Tech's website.