I don't see a relevant link in the footer: About doesn't mention ToU or link to one; nor does faq; cc-wiki and attribution is for the user contributed content. May 21, 2010 at 15:52
1) All the data is CC-Wiki+Attribute
2) This says you may use the logo to ID where the data is coming from but your app must not have the same look and feel or use any of the other trademarks.
3) With an API key you get 10K calls per 24hr, and I believe there is rate limiting although I don't recall the SO crew publicly stating what it is.