I'd like to suggest XenForo include LinkedIn integration in the core product, mainly because I believe anything related to signup/registration should be handled by the core, rather than third-party products.
LinkedIn is gaining traction across the Internet, especially for those who run forums targeting certain professional occupations.
Note: this has already been suggested, but as interest has recently grown and as per Mike's post I've made a new thread.
P.S. if you want to see Yahoo! authentication, head over to this thread and like the first post.