Jeff seems to have put it very well in his blog. Much better than I did myself back in December 2006 when I blogged about my "Onion" idea.
My mistakes were
01: I was missing a domain services layer.
02: I tried to explain it using a specific example (wizard app with a "process stack" aka "task stack"), and later decided I didn't like the example :-)
03: When I drew my diagrams I showed them as a typical stacked diagram, because my graphics skills are crap :-)
Here's my badly put idea of onion layered applications:
Well done Jeff, very nicely put!