Danger's Sidekick device uses a Java operating system, while Yahoo is known for its extensive use of open-source software to power its services.
Speaking at the Mix '08 conference, Ballmer said that Microsoft would likely take on some of the open-source PHP applications that Yahoo relies on for its services, again assuming Yahoo ever starts returning Microsoft's phone calls.
"We should not have two of everything," Ballmer said. "We'll have to sort some of that through."
He said that undoubtedly Microsoft will choose to go with Microsoft-developed services in some areas and Yahoo-developed ones in other areas, meaning that Microsoft will have open source-powered services for some time.
"I'm sure a bunch of them will be running at high scale and in production for a long time to come," he said.
On Danger, Ballmer said that Microsoft was attracted to the way the Sidekick gets applications and data to its users as well as the consumer interface and appeal. However, he made clear that he believed those things could be brought over to the Windows Mobile operating system and still preserved.
"The Danger acquisition is really about building up an application and service aspect on top of our Windows Mobile platform," he said. "Danger is really a service application experience and we want to make sure we get that in market on a great set of phones."
Ballmer addressed a ton of other topics, some of which I'll also try and get to here. Asked by an audience member "What about Adobe?" Ballmer replied "What about Adobe?" before going on to say that Microsoft will interoperate in some areas and compete in others, but made clear that Microsoft wants to win a greater share of developers' attention.
"We're going to try to give you exciting choices and encourage you in ways that make sense to pick the Microsoft alternative," he said. Asked about the browser market, Ballmer reiterated past comments that the company made a mistake in trying to tie too many products to Longhorn, a move that hurt the company in the browser area among many others.
"That was a painfully long gap," he said. "You won't see those kinds of gaps on Windows." He also said that Microsoft has learned how to incubate new browser features apart from Windows releases and then bring them back into the next version of the operating system. "We're now having to hustle as hard as we've had to hustle to drive browser innovation."