Car-sharing service Zipcar (ZIP) is having a huge first day on the public markets, with its stock climbing 70% above its $18 per share IPO price at one point. As of last check, the shares were at around $28.58, which puts the Cambridge, Mass.-based company’s market cap at a whopping $1.1 billion

Pretty amazing for a company that booked at $14 million net loss in 2010. Apparently investors are banking on the year-over-year revenue growth (29% leap last year to $186 million) and the company’s positive cash flow. Or maybe it’s just a belief that Zipcar is at the vanguard of a burgeoning urban industry that incumbant rental companies have struggled to crack.

For the uninitiated, Zipcar has a fleet of around 8,500 cars spread out in dedicated parking spots over 14 metro markets and more than 200 college campuses. Users rent cars via the Internet or their mobile devices, and then use their personal “Zipcards” to unlock the doors. Just drive and return to the same spot. Both gas and insurance are included in the rental fee.

According to an interview I posted yesterday with the CEO of rival car-sharing company RelayRides, Zipcar is believed to have penetration of between 5% and 10% of licensed drivers in its metro markets.

Prior to going public, Zipcar had raised over $38 million in VC funding, from Revolution Living (21.45% pre-IPO stake). Benchmark Capital (11.9%), Greylock Partners (6.71%), Smedvig Capital (5.24%) and Globespan Capital Partners. Of those firms, only Smedvig Capital sold some of its shares via the IPO. The rest will be prevented from selling any shares for the next 180 days.