I have read many, many times on TripAdvisor that restaurant.com certificates have no expiry date.
I purchased 2 $100 certificates back in April - 1 for StripBurger and 1 for BB Kings. We were planning to go back to Vegas in Feb, this has now been postponed for a year till Feb 2013, or maybe even later that year.
It just seems to good to be true that my gift certificates will still be valid 2-3 years after I bought them!!