Barry Schwartz posted over on Search Engine Land about Google's Onebox Answers becoming more robust. [searchengineland.com
These are not simple one sentence answers. These answers are a full paragraph with many details.
I have been looking into it and it seems like Wikipedia is the source for most of the ones I found. I have also seen these sites being used:
SearchEngineLand.com > Search for "What is SEO"
MRC.ac.uk > Search for "What is equality"
MattCutts.com > Search for "Who is Matt Cutts"
Wikipedia.org > Search for "What is google+"
BusinessInsider > Search for "Who has played batman"
Pubcon.com > Search for "What is pubcon"
These results are on google.com in USA. If you are not seeing them for samples listed above, try adding &gl=us to the address bar and repeating the search. Mods Note: We are making a one time exception to the forum rule about not posting specific examples. The samples listed above should allow us to discuss this without needing to mention any other specifics.