TargetSpot entered the mobile advertising world this week with an expanded partnership with Slacker Radio to include audio ads in Slacker’s iPhone, Android and BlackBerry apps.

Slacker Radio is the first announced customer for TargetSpot’s mobile audio ads, but it is in talks with other partners, says Eyal Goldwerger, TargetSpot CEO. With cell phones being audio by nature, TargetSpot figures its audio ads are a perfect match – and advertisers don’t need to make new creative if they want to use what they’re already doing on the desktop.

TargetSpot started working with Slacker in 2008 to sell Slacker’s audio ad inventory and will now serve audio advertising to Slacker’s mobile applications on the aforementioned smartphone platforms. TargetSpot’s technology, however, can work on virtually any phone.

TargetSpot has been in the online advertising business for some time with a lot of large partners, including Yahoo! and MySpace Music. “We believe, like most people out there, that mobile advertising is going to be a very big opportunity,” Goldwerger says, referring to various research firm estimates on the size of the mobile ad market.

But much of the focus is on display ads, search, text and video. “When you think about the phone, it’s really an audio medium to start with … Our focus is very much on audio and that’s our core strength,” dynamically inserting ads into the stream.

To his knowledge, TargetSpot is the first offering this kind of mobile advertising solution, but Goldwerger is careful not to make blanket statements about what other companies known or unknown may be up to. It took the company years of development and refinements to reach this point.

How tolerant are consumers to the ads? Goldwerger says that’s the easy part. The commercial radio is many decades old and people are used to hearing ads on the radio. As long as advertisers adhere to the common practices of frequency, i.e., not running the same ad too many times, users are fairly tolerant of the short audio ads. It’s a passive medium so it takes no effort on the listener’s part unless he or she wants more information.

TargetSpot has about 30 employees and its investors include CBS Radio, Union Square Ventures and Bain Capital Ventures. Online, the company has more than 70 partners.