Salesforce hosts an annual conference for its customers called Dreamforce. At the Dreamforce conference in November 2009, there were a couple of presentations on user adoption.

One session was called The Cardinal Rules of User Adoption (PDF available; 38 slides). John McGuigan of Fiberlink Communications presented their approach to user adoption for Salesforce, and included this excellent chart (see page 12 of the PDF) about the short-term and long-run effectiveness of difference philosophies of user adoption:

The whole deck is worth a read.