[lwan]

Well it's pretty obvious, isn't it? In order to prove that you were a patron, you can just provide a unique receipt. Yelp can be the anonymizer.

Of course, this requires so many additional layers of complexity (e.g. unique codes, local Yelp POS software, etc)that Yelp would never actually use it--it simply increases the barriers for writing a review. I guess the trick is in implementation. Maybe verifying purchases through transaction clearinghouses?