The YaST source code is kept in an open SVN repository on opensuse.org. YaST is one of the first projects that moved to the openSUSE SVN server.
The main development takes place in trunk. This is its repository URL:
After a product release we create a branch, so that there is for example for openSUSE 11.0 the following branch:
Find as well a list of all current SVN branches.
To get a current checkout type on a commandline (change "yast-svn" to whatever you like):
mkdir yast-svn cd yast-svn svn co http://svn.opensuse.org/svn/yast/trunk/ cd trunk
With the checkout on your hard disk you may want to to build on your own.