From the “why didn’t that happen years ago?” department, Yelp announced today it will integrate OpenTable reservations. According to the Yelp Blog:

Logged in Yelp users are now able to easily book a reservation using this feature and you don’t need to have an OpenTable account. Already have an OpenTable account? Groovy! If you used the same email account to set up your Yelp and OpenTable accounts — you’ll automatically receive credit to your OpenTable account, for your reservation — no need to log into both sites. We’ve got your back.

That covers the user perspective. From the restaurant perspective, those that are already part of OpenTable’s system will automatically get the new feature on their Yelp listing. In this way, Yelp joins OpenTable’s affiliate network that also includes Zagat, Yahoo, MenuPages and TripAdvisor.

This should boost Yelp’s appeal as it sees growing competition from a slew of mobile local services as well as existing online competition like CityGrid Media (whose Urbanspoon subsidiary just launched OpenTable competetor “Rez”). It will also presumably help OpenTable through more exposure and possibly sales through Yelp (speculation).

I ran a panel at our Marketplaces conference in March that included executives of Yelp and OpenTable. They had quite a rapport with each other, which makes sense … as does this deal.