Just when we were talking about it... thanks Marco:

Mobile Link Discovery Specs

These days a lot of web sites have mobile optimized webpages and usually publishers use different URLs for them. For example, Google has its mobile version of homepage at http://www.google.com/xhtml (instead of http://www.google.com/).
Usually publishers (Google in this case) uses techniques to dispatch mobile devices to its mobile version by looking up User-Agent string. This works fine when you make mobile webpage that only links to web pages under your control.
However, this might cause a problem linking to external sites from mobile optimized web pages. We can't make sure the target URL has mobile optimized page or not. Mobile Link Discovery tag solves this problem.

<link
rel="alternate"
media="handheld"
type="application/xhtml+xml"
href="http://yoursite.example.com/mobile" />