Ballmer sidestepped a request to make any specific comments on the Android software platform during a Tokyo news conference Thursday. "Well of course, their efforts are just some words on paper right now; it's hard to do a very clear comparison [with Windows Mobile]," he said.

Ballmer noted the success his company has had with its Windows Mobile platform, which commands a sizeable share of the smartphone market, especially in North America. Windows Mobile is on 150 handset models and available from over 100 different mobile operators, he said. Microsoft will likely license 20 million Windows Mobile handsets this year, he added.

"We have great momentum, we've brought our Windows Mobile 6 software to market, we're driving forward on our future releases and we'll have to see what Google does," said Ballmer. "Right now they have a press release, we have many, many millions of customers, great software, many hardware devices, and they're welcome in our world."

Android was developed by Google and others under the umbrella of the "Open Handset Alliance." The Linux-based platform will combine open-source components and include an operating system, middleware stack, customizable user interface and applications.

Big names in the wireless industry have already thrown their weight behind OHA, including carriers such as T-Mobile, NTT DoCoMo, Sprint Nextel, Telecom Italia and China Mobile; handset makers including Samsung Electronics, LG Electronics, Motorola and HTC; and others such as Intel, EBay, NVidia, Qualcomm and Texas Instruments.

Alongside Windows Mobile, the biggest competitor Google will face is Symbian. Two days earlier at a news conference in Tokyo, the CEO of Symbian also downplayed the platform's significance.

"One of the reactions is, it's another Linux platform," said Nigel Clifford. "There's 10, 15, 20, maybe 25 different Linux platforms out there. It sometimes appears that Linux is fragmenting faster than it unifies."