I first noticed the problem in Salt Lake, where a bar was leading the vote for several weeks. Not a romantic one by any means. After discussing with many of Urbanspoon's prime members (users who make significant contributions in each city) it was discovered that the problem was very widespread, and frequently due to restaurants inflating the votes themselves by encouraging customers and employees to vote for them. Now I totally condone restaurants asking customers to make their vote, or write a review on these sites. But employees are definitely crossing an ethical line, and asking for a vote that is truly undeserved is not only unethical, but it will backfire on the restaurant, the site, and the credibility of the users.
Shill reviews are always a bad idea. A shill is a positive review for your own business, or posting a negative review for a competitor. People often take advantage of the anonymity of the web, thinking they can say whatever they like with no consequence. The problem is that it is really easy to spot a shill, and once discovered you will feel a worse fate than just a few bad reviews.
In regards to this Valentine's Day contest, if someone looked to this poll to make a decision on a date-spot, then found themselves at a very unromantic place, it's not going to sit well in their minds. There are few experiences in life that carry worse feelings than a failed date. When a restaurant is a factor in a bad date, the customer typically won't give that spot another chance because of how disappointed they were on their visit. The total experience matters when customers are trying a new business, and a bad first impression is a lasting feeling that is not easily cured.
You may think it's all out of fun, or believe that some good attention may get you new customers, but if you get customers under false-pretense the backfire can spread quickly and will turn that target audience against you.