YQL will currently fall under the standard Yahoo! TOS agreement. Other than the TOS, the YQL documentation is all you'll need: http://developer.yahoo.com/yql/. This is the currently available information. Should these TOS guidelines for YQL be revised in the future, the information will be posted on the developer.yahoo.com site.
Jonathan LeBlanc Senior Software Engineer Yahoo! Developer Network
Phil, thanks for the question. We're really happy that you'd like to use YQL in your commercial application.
To add a little more information to Jonathans answer, at this point Yahoo! is making YQL available "as-is" and without warranty or commitment to provide developers notice of changes to or availability of YQL from Yahoo!