We don't have a built-in way to do that, but there are ways you could accomplish that on your own site.
The simplest way would have the product's purchase button actually be just a regular link to your Terms page for the item, where the actual Add to Cart or Buy Now button would be at the bottom of the agreement and the standard button image replaced with a custom button or text that says Agree or Accept.
Other sellers have written custom JavaScript for their page that simply disables the purchase button until the agreement box next to it is checked. Offhand, I know CommonCraft.com had done something like this before, although their site has gotten even fancier with custom implementations of our cart since then, but you might contact them to see if they'd be willing to offer any tips.
Otherwise, you could inquire with any of the competent, E-junkie-experienced developers listed in our directory:
http://www.e-junkie.com/ej/developer-directory.htm
