The answer depends on which source code you are after.
The Downloads page contains links to:
- Stable Release - strongly recommended
- Development Release - should not be considered stable
- Nightly Builds - latest and greatest
Now the strange things about releases is that they do not include all the code. Some modules are not considered ready at release time, and are thus not included even in source form.
You can grab 2.1.M2 using subversion as described in the developers guide:
Or nightly builds:
Or you can grab trunk: