Placecast has its own version of the invisible fence, except these geo-fences don’t protect passing consumers from retailing hounds — or at least they don’t protect consumer wallets. When consumers who have opted in are within a geo-fence surrounding a participating retail outlet, they may possibly receive SMS messages notifying them of sales or detailing offers.

Now Placecast partnered with O2, the UK’s second largest wireless carrier with 22 million customers, to bring location-based marketing to its O2 More opt-in service, in which wireless customers to receive promoted messages from brands based on age, gender and interests. Starbucks and L’Oréal are the first brands to take advantage of the six-month trial period — when O2 More subscribers walk into more than 1,500 geo-fences surrounding Starbucks and L’Oreal locations in the UK, they may get a message enticing them to come in and buy.

Placecast claims this is the United Kingdom’s first location-based mobile marketing endeavor, and the largest geo-marketing campaign undertaken.

“The market potential here is huge,” commented Shaun Gregory, managing director of O2 Media, the carrier’s marketing arm. “It has already had massive success in the USA — and this is another nail in the coffin of the old model of reaching mass audiences in one go.”

Through using opt-in geo-fences, virtual boundaries that can be targeted using location-based marketing, retailers can reach out to customers with customized messages based on sign-up preferences when they’re near a store or actually inside it. Offers will appear on mobile websites or social networks such as Facebook, or can be sent via SMS for non-smartphone users.

Pilot programs with fast-food chain SONIC and retailers American Eagle Outfitters and REI had promising results, with 65% of surveyed participants making a purchase as a result of a ShopAlerts message. Nearly 80% of respondents said ShopAlerts increased their likelihood of visiting a store while 73% said that would likely use the service in the future.

Here’s a dramatization of how it all works:

Its targeting algorithms correlate a user’s expressed interest in real estate locations with dozens of other demographic, psychographic and geographic data points.

“Once we know the place a user is interested in, we can derive a lot of useful insights about what kind of consumer they are, and then serve them a very targeted ad,” said Alistair Goodman, CEO of Placecast.

For publishers in categories such as travel, events, tickets, real estate, weather, classifieds and career search, Placecast uses the location information already present on these sites to target audiences and customize messaging.