WebKit (Chrome, Safari) does not support transitions on pseudo elements.
- https://bugs.webkit.org/show_bug.cgi?id=23209
- http://code.google.com/p/chromium/issues/detail?id=54699
It should work in Firefox.
Edit: The issue in WebKit is now resolved. The patch allready landed in Chrome Carnery, so it will be supportet from version 26 on. I don’t know about Safari.