Andrew Niefer announced the availability of the Eclipse 4.1 M3 builds.
You can download it from http://download.eclipse.org/e4/sdk/drops/S-4.1M3-201010291118.
The e4 build provides a repo that can be used to install the source bundles for e4 UI and e4 UI CSS, as well as other e4 functionailty like the modeling tools, XWT, and SFS (resources). This has been a big problem for Eclipes 4.1 M2 and seems to be solved now. Congrats for providing the p2 update site!
Unfortunately the p2 update in Eclipse 4.1 M3 takes very, very, very, very long. So be patient if you try to install the e4 tooling.