Eric Ly, a co-founder of professional networking site LinkedIn, has signed on to be a venture partner at Wellington Partners. Ly says he’ll be helping the London- and Munich-based firm with its plans to expand into the United States, and into Silicon Valley in particular.

That’s a natural expansion for Wellington, given its interest in technology and the current Euro-dollar exchange rate. But it’s a surprising move for Ly — after all, you’d think he’d have his hands full with his scheduling startup, Presdo, where he is the founder and chief executive. (I’ll be writing more about Presdo soon.) And indeed, Ly says that Presdo is “my life”; his deal with Wellington only commits him to working two days per month.

“The first time they approached me, I said no,” Ly says. “But [General Partner Eric Archambeau] was just very persistent, you know, and I started to see some benefit in doing this part-time, for myself.”

Although Ly says his heart is still in startups and entrepreneurship, the deal gives him a chance to dip his toes into the venture world. Archambeau’s credentials as an investor were a big draw too — before joining Wellington, Archambeau worked at Benchmark Capital and Atlas Venture, and previous investments include eGroups, which became part of Yahoo Groups, and Trading Dynamics, which was acquired by Ariba.

At Wellington, Ly will focus on social media and social networking. With a limited time commitment, it’s hard to imagine that he’ll have a huge effect on the firm’s direction, but his name is an impressive addition to Wellington’s stable of venture partners, which also includes Loic Le Meur, chief executive of Seesmic (a Wellington investment), and former Fireclick chief executive Ram Sranivasan.