Add A Terms & Conditions Checkbox before Checkout

Hi All,
Is there a way to add a terms and conditions checkbox (or a terms & conditions plugin that plays nice with MarketPress)? I love MarketPress - it works well with the Thesis Theme and is easy to install etc. I need figure a way to add this checkbox at some point to the site (ie. I could force people to register before purchasing I suppose and then add it there, but would prefer to add it before checkout) I'm working on...
Thanks!
Kate