pierrefar - 1:43 pm on Jun 20, 2011 (gmt 0)
To answer a couple of questions:
1. We currently support the rel="canonical" HTTP headers for web search only, and we're keeping an eye on how webmasters use them to see if we should support other Google properties.
2. The rel="canonical" HTTP header does work across domains, just like a link rel="canonical" link tags in HTML.