For multiple Github repos, use http://bazel.io/docs/build-encyclopedia.html#http_archive or http://bazel.io/docs/build-encyclopedia.html#new_http_archiv... (depending on if it's a Bazel repository or not). Let us know if you have any questions or issues!
A couple more questions :)
* Any pointers for adding Scala (sbt?) support? I'd start here: http://bazel.io/docs/skylark/rules.html.
* Suppose I develop using multiple repos and http_archive. I'd like to make changes both to a library and to a project that depends on it simultaneously, without committing the library patches to master github repo just yet. Is there a way to configure the http_archive, let's say by saying "bazel --mode=local", and have it customize the remote archive http to use a different url (say, my github's fork instead of the master github) for that build?
For multiple repos: there's no command line flag, but you could change the WORKSPACE file to use http://bazel.io/docs/build-encyclopedia.html#local_repositor.... Unfortunately, this may be of limited use to you. At the moment it's optimized/bugged to assumed that your local repos don't change, so it won't rebuild them (this is great for things like the JDK and gcc, but not so much for actual in-development repos). Feel free to file feature requests for any functionality you need, I'll be working on this a lot over the next couple months.
* If I have a Maven-based project with heavy reliance on pre-built jars from Maven Central, what's the recipe to port it to Bazel?
* Related, if I have multiple github repos, say a couple open source libraries and a couple private repos, what's a good recipe in conjunction to Bazel?