The APIs Discovery Service launched today. This has been in the works for a while, but probably not a secret if you've been following checkins to the google-api-python-client library. The announcement went out on the Google Code blog, and has been covered in other venues.
Two things to add that haven't been covered so far, the first is that if someone besides Google were to build an API that could be described by the Discovery format, then potentially some of these libraries would work with that API. Secondly, check out that patent license. While the obvious questions will be raised, instead let's fill the comments with your theories of why we didn't use WADL.