As for the license the code is under, I say "Yes and no" and detail them both. The "no" part address the fact that Readable's code isn't (and probably won't be) in any public code repositories.

If you're interested in the code, though, you can find the bulk of it here: http://readable-static.tastefulwords.com/_r/bulk.js

And, yes, it is open source; currently under a variation of the Creative Commons License; but, as detailed further up in this thread, that'll probably change by tomorrow -- as, apparently, Creative Commons isn't recommended for source code.