Zipcar, the world’s leading car-sharing service based in Cambridge, issued two back-to-back press releases this morning. The company filed for IPO in June of this year, and everyone — especially here in Boston — has been keeping a close watch on it since. Today’s releases suggest that the company may be delaying its IPO.

First the company announced a Series G round of $21 million. California-based Meritech Capital Partners specializes in expansion-based investments and contributed $20M to the round. Pinnacle Ventures, also California-based, contributed the additional $1 million. Currently Zipcar has over 500,000 members and 8,000 vehicles in its fleet in the US, Canada and UK, and the financing will be used to expand and ramp up this fleet considerably.

The second release announced the addition of two seriously accomplished board members: Steve Case and John Mahoney. Case is co-founder and former CEO and Chairman of AOL. He is also the founder of a holding company with a controlling interest in Flexcar, which Zipcar acquired in 2007. Mahoney is Vice Chairman and CFO of Boston-based Staples. Case will take a former board member’s seat, and Mahoney will expand the board to 9 members.

One research firm has projected world-wide car-sharing revenue could hit $3.3B in the next five years, and Zipcar holds about three-quarters of the global car-sharing market (read about where Zipcar got it right here). Car-sharing is primed for high-growth, particularly given the global eco-consciousness and green trends. For example, Boston-based Relay Rides just announced West Coast financing yesterday, and will be moving its headquarters to San Francisco.

How long will this recent injection of working capital keep Zipcar financed before needing to put their foot on the IPO gas pedal?