From the Symfony website:
If you are a reader of TechCrunch, Read/WriteWeb, or ZDNet you may already know that there is a preview of the next version of del.icio.us. What you might not know is that the next version of Delicious is built with symfony. Of course, Yahoo! extended and modified symfony to fit their needs, but what's great is that they could actually do it, and that they will contribute their modifications back to the community in the form of plugins and bug fixes.
See more here.
This is obviously great news for all us Symfony developers. If you're a PHP'er I would highly recommend checking out this great framework.