Twitter currently allows marketers who send out promoted tweets to target users based on their interests, gender, device, and even keywords. Twitter also has some form of geo-targeting – but it’s not too specific. Twitter only lets promoted tweeters target by country or metro area.

But that may be about the change.

Ad Age quotes two sources familiar with the matter who say that Twitter is developing a hyper-specific geo-targeting ad product that will let retailers catch users’ attention when they pop into the vicinity of their stores.

Apparently, the new ad product could launch as early as the end of the year.

It could start off with zip code targeting – something that rival Facebook has had for a while. After that, the targeting could get even tighter.

It’s obvious how this sort of specific geo-targeting could be a very important new tool for advertisers. Retailers could catch users as they pass certain stores and restaurants. Users could be alerted to any sales, deals, or special offers available at certain stores via promoted tweets. And marketers could reduce “waste” in advertising by only targeting a very specific geographic area with their promoted products.

For Twitter users, yes, it’s more highly-targeted ads. But when you think about it, promoted tweets containing offers or deals that pop up in your stream when you’re nearby could be looked at as a feature, not just as advertising.

Sometimes it’s hard to see the difference.