Dominik Homberger published a new version of libwebp at the beginning of the year. Take a preview at http://libwebpjs.hohenlimburg.org/v0.2.0/.
This new version supports alphachannels, lossless compression and can be merged into HTML5 easily.
Now, I integrated this library as experimental feature into wp-webp. the WordPress plugin to combine WordPress and WebP format.
You will find the settings under Options -> General.
Looking forward to your feedback.🙂