The mobile device company responsible for the Sidekick smart phone, and originally co-founded by Andy Rubin -- who now heads up an open source project at Google -- will be joining the Microsoft arsenal in the mobile space.

Although stymied for the moment in its multi-billion dollar bid for Yahoo, Microsoft forged ahead over the weekend in building up its cache of buyouts, leveraging the remote venue of a mobile conference in Barcelona, Spain to announce the purchase of Danger, Inc., the company that created the Sidekick smart phone.

"With all the excitment about what's going on in the company right now, [this deal] is critical to our future and decisive to our future," stated Robbie Bach, Microsoft's president of entertainment and devices at a news conference at the Mobile World Congress in Barcelona this morning.

The Danger buyout isn't exactly a hostile takeover, although one of Danger's co-founders, Andy Rubin, is currently director of mobile platforms at Google, the traditional archrival to both Yahoo and Microsoft in Web-based search and advertising.

In contrast to the highly publicized negotiations in Microsoft's try at buying Yahoo, Microsoft is keeping a lid on the selling price for Danger.

But the deal is seen by many observers as opening up opportunities for Microsoft in mobile applications, an area that also happens to fit in well with Microsoft's ambitions toward advancing its display advertising capabilities on both mobile and other Web-based consumer entertainment platforms.

Bach reportedly contended that the Danger acquisition "completes the picture for us in terms of making the transition from just being on the business side of things to being on the consumer side of things."

Beyond the buyout of Danger and the mobile ad deals with European publishers, Microsoft also announced at the show that it's gotten the nod from Orange, a major mobile operator, to serve as its ad serving partner for mobile display ads in Spain. And Microsoft also announced that several major European mobile publishers -- including Autonews, L'Equipe, and Boursier.com -- are now using Microsoft's mobile ad platform.

Yet Google has made news in Barcelona this week as well, with several of its partners demoing applications now under development for Google's Android, an open source mobile development project now headed up by Rubin. With the launch last fall of the Open Handset Alliance (OHA), 34 companies around the world announced support for Android.

Meanwhile, officials of some other companies that were not part of the launch, such as Verizon Wireless -- have since said they're in favor of an open source development environment, as a way of spurring applications that will easily run across multiple vendors' mobile platforms.