Here's how it will work, says Cyriac Roeding, executive vice president at CBS Mobile: First, you must opt in (sort of) by enabling the use of location-based services for purposes other than just emergency 911 on your phone. What such services will be called and the steps for allowing them will vary by handset and provider, but the setting will be general and won't mention ads.

If your carrier and you agree to use Loopt, the service will obtain location data from the phone, using cell tower triangulation instead of GPS. At first, only Sprint and Boost Mobile customers will get the service, via CBS Sports Mobile and CBS Mobile sites viewed in their phone's browser. Loopt substitutes a location-based advertisement (for a nearby eatery, say) in place of the potentially less-relevant ad that would otherwise appear on those pages.

Now for the privacy measures: Roeding says that the Loopt/CBS Mobile process won't associate phone location data with the user (by sending a phone number or account name with the location, for instance) and that the service won't save anyone's location data.