It's possible capture and put in the tool tip or label the R-Squared from trend model?
You have to calculate it yourself:
Here's one post that gets to a possible solution: http://community.tableau.com/message/184294
It's also an idea on the community:
http://community.tableau.com/ideas/2229
In upcoming versions of Tableau, you will be able to use R to get to these types of statistical numbers.