Hi Dave,
Most websites that have a review structure frown upon giving something in return for a review. Yelp is very clear on these guidelines:
http://officialblog.yelp.com/2009/09/to-solicit-or-not-to-solicit.html
On the Google site, this is known as a conflict of interest:
"Reviews are only valuable when they are honest and unbiased. Even if well-intentioned, a conflict of interest can undermine the trust in a review. For instance, do not offer or accept money or product to write positive reviews about a business, or to write negative reviews about a competitor. Please also do not post reviews on behalf of others or misrepresent your identity or affiliation with the place you are reviewing."
Source: http://www.google.com/support/places/bin/answer.py?hl=en&answer=187622
I hope that helps!