I’m an avid ebay shopper, but I just noticed that ebay is using sub-domains for its main categories (left side of homepage). I think this is a new development, and one could assume that ebay is trying to leverage the sub-domain strategy for better organic search rankings. Of course about.com is famous for utilizing sub-domains, and it ranks for everything under the sun.
I’m often asked at work if sub-domains are a good way to optimize a site. I tend to recommend using sub-domains when the content and categorization makes sense, but not so much that the site becomes fragmented and looks unnatural. For instance, ebay has long had sub-domains for motors and express, which makes sense because they are separate from the main ebay shopping site.
It will be interesting to see if this change makes a positive or negative impact on the site’s traffic.