The Wall Street Journal is reporting that Google has been investing “hundreds of millions of dollars” in a cellphone project. Reports of a “Google phone” are not a particularly new revelation, but this latest report comes with some added details.

The Journal reports that Google has made mobile phone mock-ups, courted carriers (T-Mobile, Verizon), and continued working on mobile applications. They also report that the finished phone would be available next year at earliest, according to “people familiar with the idea” (the Journal made numerous references to such sources). Earlier reports have suspected Andy Rubin, co-founders of the mobile device manufacturer Danger, has been working on a mobile device and operating system since selling his startup Android to the search giant.

Advertising is cited as the main motivation behind Google extending its reach to the mobile phone, an with good reason. CEO Eric Schmidt has been quoted saying mobile ads are “twice as profitable or more than the nonmobile phone ads because they’re more personal”.

While Google already has a solid lead smartphone search traffic, Google still has to play within the guidelines of the mobile incumbents. Google’s hopes to simply go-it-alone were recently dashed after the FCC’s recent $4.6 billion plea for an open 700 Mhz spectrum were shot down. Instead they have been working with carriers to ensure their mobile applications come bundled with the phones, as they have most recently done with Sprint. Google has also done a deal with the United Kingdom’s Vodafone Group PLC to provide the search bar on the carrier’s branded Internet homepage.