Wolfram does include a source listing for its entries, but there's no indication which specific facts come from where. A rather spare entry about Napoleon Bonaparte included (when I clicked on "Source information") a general list of references that included both the Encyclopedia Britannica and Wikipedia. But the primary source was "Wolfram/Alpha curated data."

Final verdict: Wolfram|Alpha is fun to play with, and introduces an original format for finding information on the Web. As it currently stands, it may be useful for mathematical and scientific research. However, until it aggregates a lot more data, and learns to handle more complex user queries, this interesting experiment is no danger to Google. In this case, David is lobbing equations at Goliath instead of stones -- which may be impressive, but certainly won't kill any giants.