CEO Eric Hertz will be leaving Zumobi, the Seattle company developing a mobile phone user interface supported by advertising, we have learned. His last day will be Aug. 1. Taking over the reigns is Ken Willner, who joined the company in February as VP of market development. Previously, Willner worked at AT&T (NYSE: T) Wireless, where he served as VP of Advertising and Media and worked on the American Idol marketing campaign. Before that, he worked in the advertising industry, where he started Fizzylab, a contextual search and advertising company. Hertz, who is an investor in the company, told us he will remain on the board. “This is more of a decision about me than the company. For my professional career, I felt the need to take a look at some other things,” he said. “It’s a great transition for the team, I wouldn’t have invested and wouldn’t have wanted to stay on the board, and help make it something, if that wasn’t true.” 

Zumobi, formerly called ZenZui, was spun off from Microsoft (NSDQ: MSFT) as part of Microsoft’s IP Ventures program with Zumobi owning the rights to the intellectual property and Microsoft continuing to own a stake. The company has raised about $12 million from Oak Investment Partners and Hunt Ventures. Since launching at CTIA last year, Zumobi released a version of its software for Windows Mobile, and last week launched on BlackBerry. Hertz said he wasn’t leaving because he doesn’t believe in the company, but conceded that “as with any technology, it takes longer than you expect, and you end up in a different place than you started out going.” It wasn’t the number of companies that entered into the space that caught him off guard, but it was the iPhone. “It’s not so much the iPhone as a product, but the whole relationship between the handset maker and the carrier—it’s completely reoriented the whole user-experience and the relationship with the customer. And that was huge. That was more of a surprise.” Prior to Zumobi, Hertz was COO of Western Wireless before it was acquired by Alltel (NYSE: AT). He does not know where he will go next. “You put 200 percent into this, and you can’t think about changing careers…I just needed to create some space.” Will he go back to a carrier? “Being on the other side of the fence that has such a negative connotation, but I think what I’m looking for is not necessarily to be at a carrier, but on the service side of the business.”