Palm unveiled a new smart-phone Thursday that will be available next month for about $100. That’s the lowest introductory price yet for a Palm handset and will make the phone, dubbed the Centro, the least expensive smart-phones on the market.

Despite having a different name from its Treo siblings, the Centro will share with them the Palm operating system, a color touch screen, a full keyboard and the ability to access e–mail and surf the Web.

Initially, Palm will offer the phone exclusively through Sprint for about three months, and it will be able to tap into the carrier’s high-speed EVDO network. The phone can be bought for $100 after a $100 mail-in rebate and is available only to customers who sign up for a two-year service agreement that includes data access plan of more than $25 a month.

Although rumors of the Centro’s imminent launch have been circulating for weeks, investors cheered its official debut, sending Palm’s stock up as much as $1.02, or 6.6 percent, in trading today. While Palm jump-started the smart-phone industry with its Treos, the company has struggled in recent years amid competition from Nokia, Motorola, Research in Motion and Apple. The Centro is the Sunnyvale company’s first new U.S. product since it killed its Foleo notebook-like device earlier this month.