Here are links to the slides from some darcs-related talks given by David Roundy:
FOSDEM talk from February 2006. "Implementing the darcs patch formalism ... and verifying it"
This talk covers the patch formalism and the use of GADT type witnesses to enforce patch relationships.
There's also a video (270M) of the talk (albeit a bit blurry) available.
Haskell Workshop talk from 2005. "Darcs: Distributed Version Management in Haskell"
Talks about experiences of darcs with the Haskell language.
CUFP talk from 2005. "The Myth and Reality of using Haskell in the 'Real World'".
Addresses experiences with finding contributors for darcs, given that it's written in the Haskell language.
(Note: Anyone who cares to would be welcome to write better summaries...)