Placecast is announcing that its ShopAlerts platform will power location-based push marketing for O2 in the U.K. This is its first major carrier deal, joining existing deals with U.S. brands such as The North Face.

Placecast (see past coverage) divides the world into circular map tiles known as geo-fences. Depending on the geo-fence you enter and the interests you’ve indicated (by category), targeted promotions are sent via text. In this case it’s carrier delivered MMS.

This makes it an interesting mobile “discovery” play, and the opt-in gives it a nice mix of push and pull. This combination we believe represents an important evolutionary path for LBS and many mobile successes — like Foursquare, Loopt and Aloqa — employ some version of it.

The beauty of this deal is that it plugs into O2’s existing “More” ad platform, which translates to 1 million already opted-in users. Previously this used carrier-obtained demographic data and interests, but combined with Placecast’s geo-fencing, it gains some good targeting mojo.

Placecast specifically brings to the table 1,500 new geo-fences around brand locations such as Starbucks and L’Oreal. This should raise relevance and ad performance for O2 and its advertisers, while giving Placecast a major coup in its first carrier deal among many we expect.

Placecast CEO Alistair Goodman will be on stage during our upcoming Interactive Local Media conference in December to tell us more.