Thanks - the problem is that the remainder of the page is also likely to be somewhat duplicate - it's mostly NAP data, business tagline or coupon offers, which are likely to be extremely repetitive as these businesses often have dozens of listings, all with the same info.  The reviews are really our only differentiating content from all their other listings.

If the reviews popped up on their sites as well (via our widget) would a link back to their profile page (part of the widget) be enough to indicate to Google that we were the original source?  I don't think canonical tags will be an option.

But would I be able to get the canonical tag on the client website via the widget code?  Or I'd need two separate code snippets for installation

Can the canonical tag be applied to only a portion of a page's content?  (basically, everything within the review widget?)

My website has a profile page for each of the home improvement contractors who signs up.  They accumulate reviews on those pages.  I want to build a review widget to display our reviews on our contractors' websites.  How can I avoid duplicate content issues for the reviews?

Example of a profile page with a few reviews:  http://www.thehomefixers.com/members/425/waterheaters-plus-llc.php