The Google phone is like the Roswell UFO: Few outsiders know if it really exists, but it has a cult following.

Just months after Apple’s iPhone mania gripped Silicon Valley gadget heads, suspense is building over reports that Google Inc. plans to release its own cell phone.

The blogosphere is buzzing with rumors that the search giant might announce Linux-based mobile software as early as this week and a Google phone, which observers have cheekily dubbed the “GPhone,” by early 2008.

The latter is the most tantalizing to Silicon Valley, which is just getting over the June 29 launch of Apple Inc.’s multipurpose iPhone.

No one has displayed indisputable proof the GPhone exists. But one thing is certain: Google, which made nearly $11 billion in 2007 from Web advertising, is investing heavily to target the potentially lucrative and hotly contested mobile- search market.

The vision: mobile-phone service offered free of monthly charges to consumers willing to put up with advertising.

The goal: for Google to broker advertising on mobile phones the way it has on the Web.

The fear: Wireless carriers worry that Google will muscle its way into the young market and capture their wireless advertising dollars.

The official line: “Google doesn’t comment on rumor or speculation,” company spokeswoman Erin Fors said.

In recent months, the Mountain View, Calif.-based company is rumored to have aggressively sought to partner with mobile carriers and manufacturers to make its search engine, maps program and other software available on more mobile handsets and networks.

If Google does make an even more forceful move with Google-branded handsets to be offered by multiple wireless carriers, it would mark a seismic shift in the mobile industry.

“For Google, maintaining itself as a search leader as wireless Internet access grows is extremely important since this is one area with extremely high growth prospects,” said Weston Henderek, senior analyst with market research company Current Analysis Inc.

At stake is the intensifying skirmish for the mobile-phone advertising dollar.

Research company Frost & Sullivan in July estimated the U.S. mobile advertising market would hit $450 million in 2007 and exceed $2 billion by 2011. Another company, Gartner Inc., is even more bullish, predicting $3.9 billion in North America and $14.7 billion worldwide by 2011.

“Everybody is really focusing on advertising as a source of revenue, especially in the United States,” said Vikrant Gandhi, a strategic analyst with Frost & Sullivan’s mobile and wireless group.

The mobile phone is poised to become one of the most prevalent ways to access the Internet, analysts say, raising the stakes for Google. That’s why the company is exploring ways to get its services on all such devices and why it might undertake the risky but ambitious gambit of producing its own phone.

Google would not confirm whether a GPhone was in the works but said it was “collaborating with partners worldwide to bring Google search and applications to mobile users everywhere.”