It's been around a while, but I only just noticed the Microsoft Solutions Framework website because they just published their whitepapers on the Download Center (RSS feed for Download Center provided by ThunderMain - thanks guys!)
From the overview, MSF is:
Microsoft Solutions Framework (MSF) is a deliberate and disciplined approach to technology projects based on a defined set of principles, models, disciplines, concepts, guidelines, and proven practices from Microsoft. This white paper introduces MSF and provides an overview of its foundational principles, core models, and essential disciplines, focusing on how their application contributes to the success of technology projects. Finally, the paper provides references for further information about MSF and references for guidance in implementing MSF within an organization. In an appendix, the paper briefly compares and contrasts MSF to other industry methodologies and standards and describes how MSF can be used in combination with them.
MSF includes two models - team and process - as well as three management disciplines - project, risk and readiness. They also have a bunch of MSF templates you can use in your own projects.
One of the nice things about MSF is that it works with other methodologies. I.e. you don't have to give up RUP/XP/TDD/<insert your favorite methodology here> in order to use MSF.
UPDATE: Lorenzo has compiled a list of MSF links you can check out.