From the Introversion forums:
QUOTE (Chris)
Ever wanted to know what is REALLY going on in the mind of a Darwinian? Well now you can!
We are delighted to announce that the source code for Darwinia and Multiwinia is now available to buy from our online store, including immediate digital download.
Click here to go straight to our store to buy.
The code is distributed via an online subversion repository, where you can not only access the vanilla sources, but also create your own branches to share with other developers. Purchase of this product also grants you access to the Darwinia and Multiwinia development forum and wiki, where you can discuss mods with other developers.
For those specifically interested in modding Multiwinia, we are running an entirely separate Metaserver for multiplayer testing. Each copy of the Soure Code includes five authentication keys, meaning you can code changes to Multiwinia and test with 4 players and still have a key spare.
Note: You will need Windows and Visual Studio 2008 to build the games. We have tested that the code compiles correctly on the PC, but you will need to put some effort in to compile for Mac / Linux. There is no Xbox code in this release.
The source code purchase also comes with a shiny forum badge, which you can see me modelling 
(You have to log out and log back in to the forums to see your new forum badges)
This continues Introversion's long term plan to eventually release the source code to all of its games, a process started with the Uplink Dev CD several years ago. We hope to continue doing this in the future!
Have fun!
\o/
xander