"We are always excited to collaborate with anybody who wants to collaborate with us," said Ballmer, speaking at a BitKOM press conference in Cologne, Germany, according to the Dow Jones news service.
Motorola co-chief executive Sanjay Jha also said this week that his company has not ruled out working with Microsoft on Windows Phone 7 smartphones, Dow Jones reported.
Microsoft sued Motorola last week in U.S. District Court for the Western District of Washington, claiming Motorola's Google Android phones violate Microsoft patents. Microsoft also filed a complaint against Motorola with the International Trade Commission.
"We have a responsibility to our customers, partners, and shareholders to safeguard the billions of dollars we invest each year in bringing innovative software products and services to market," said Microsoft deputy general counsel Horacio Gutierrez, in a statement.
Microsoft plans to hold a launch event for Windows Phone 7 on Oct. 11 in New York. Samsung, LG, Toshiba, and Hewlett-Packard are among the hardware makers that have confirmed plans to build devices around the OS.
RIM, with its Blackberry OS, holds the largest U.S. market share for mobile platforms, with a 39.3% stake as of July 30, according to market watcher Comscore. Apple's iPhone is second, with a 23.8% share, followed by Google's 17% share. Microsoft was fourth, with an 11.8% stake.
Microsoft is hoping Windows Phone 7's tight integration with the Windows development environment will give the platform a boost out of the gate, as third-party application developers will already be familiar with many of the development tools.
The list includes Visual Studio 2010 Express for Windows Phone, a Windows Phone 7 add-in for use with Visual Studio 2010, XNA Game Studio 4.0, Windows Phone 7 Series Emulator for application testing, and Expression Blend for Windows Phone.