As Googler, Matt Cutts Twittered, he now has Sitelinks under his domain for a search on [matt cutts]. Why did it take Matt over two years to get Sitelinks for his domain when it is such an authoritative source?
The answer might be that his content was on a sub-directory. Yes, right now, there is no substantial content on www.mattcutts.com, all or most the content is on www.mattcutts.com/blog/. And it seems like Google is now giving sub directories Sitelink love.
A WebmasterWorld thread has discussion around the topic, noting that Matt Cutts is not the only query returning a sub directory listed with Google Sitelinks. I'll just guess a few right now and show you that others work, such as:
* MacBook Pro:
What does this show to us? That you can have different Sitelinks for different queries, if your site is authoritative enough. Google told us this fact in the past, but we were all skeptical, including myself, that this is possible. I guess I have yet to see Sitelinks for the same URL change based on query but it can change for different URLs on the same root domain.
Forum discussion at WebmasterWorld.