At last, a flavour of source builds are now available for Eclipse.
Since we Eclipse uses a part of Eclipse (PDE) to generate the build scripts what we have for you right now are sets of pre-generated build.xmls and fetch scripts that can fetch and build a particular version of Eclipse. This allows you to build Eclipse with only Ant, CVS and a JDK installed.
We are working on a couple of other source build scenarios.
1) A build with pregenerated build.xml files but a zip file that contains all the source files so that you don't have to do the cvs fetch. This should be up in a couple of days.
2) A source build that contains a .jar with the pde and a few other essential plugins. While this does not satisfy the "build from scratch" requirement, it doe satisfy the "build Eclipse the same way Eclipse.org builds Eclipse" requirement. This one may take a couple more weeks to put up.
So for now, you can check out the fetch and build for the 2.0 eclipse. Source builds will also be part of each every new Nightly and Integration build. Please take a look and let us know what you think.
Bugs can be logged in Bugzilla in the platform-releng product/component.