Wolfram Alpha also isn't sure what to do with this:

http://www.wolframalpha.com/input/?i=per+capita+consumption+...

It's yet another real world question that's come up in conversation that I've wanted to know the answer to, and which Wolfram Alpha would be really awesome if it could handle, but sadly cannot.

Washington, DC is the only US capital.

Alpha doesn't do any better with this one:

http://www35.wolframalpha.com/input/?i=southernmost+state+ca...

which Google seems to handle just fine:

http://www.google.com/search?q=southernmost+state+capital+in...

Surely state capitals count as capitals in the US?

Those are state capitals, not US capitals. Though I guess you could make the case that America has 3 capitals: U - S - A!