When we first heard this morning that Mark Jacobstein had left the mobile location-based startup Loopt to take an entrepreneur-in-residence position at Charles River Ventures, our initial thoughts were, Uh-oh — what’s up at Loopt? We haven’t heard much from the high-profile NEA- and Sequoia Capital-backed company since it won a deal with Sprint (S) in July. Many location-based mobile services have struggled to gain a significant amount of paying subscribers, and Disney (DIS) recently said it will end its “kid-tracking” mobile service by the end of the year.

We decided to call up Jacobstein himself, who chatted with us from Israel, where he said he was busy meeting with some “very interesting” startups. Jacobstein, always the perky company face while executive vice president of corporate development and marketing at Loopt, assured us that there’s no bad blood involved in his departure. He’s left Loopt, he said, to pursue and lead an idea of his own as an EIR at Charles River.

He’s already been on the job for four or five weeks, and said he has a dozen or so half-baked ideas he’s kicking around. Whatever Jacobstein does it will likely have to do with broadband and/or mobile, given he was also part of the founding team of mobile gaming company Digital Chocolate, and was founder and CEO of Small World Sports, widely touted as the first online fantasy sports company.

On the subject of leaving Loopt, Jacobstein was unsurprisingly reticent and urged us to talk to the current team for an update on the company’s plans. Sam Altman is the company’s 22-year-old (is he 23 now?) founder and CEO.

While Loopt has announced more wins than even we had predicted for the company, we still remain skeptical of the scalability of some of the social applications for mobile location-based services. We contacted Loopt for an update on their subscriber numbers and carrier deals and will update the post when we hear more.