It seems mashups won't grow to its potential until tools are available for the business users to easily create the data combinations they value. What are the best business-user tools available today?

We certainly agree.  JackBe works pretty hard to have mashup-creation tools that work for developers AND tools that work for business-users.  Gartner recently said in 'Cool Companies to Watch, 2009' that JackBe's strength was in its support of technical and non-technical mashup creators.