Skype already has a history of being bought. The project and company was founded back in 2003 by the same guys who did Kazaa, only to be bought up by eBay for $2.6 billion back in 2005. eBay didn't really know what to do with it though, so they sold a 70% stake to a group of technology investors in 2009.
And now, Microsoft is swooping in, even though both Google and Facebook were reportedly interested in buying the VoIP software company. It all seems a bit strange, though, since Skype never actually made any profits, and has a rather massive long-term debt ($686 million of the $8.5 billion). Still, Skype is a recognised brand the world over, and has 663 million subscribers. And, it's pretty much impossible for Microsoft to make Skype's interface on Windows worse than it already is, so there's that.
It will be interesting to see what, exactly, Microsoft's plans are for Skype. Due to carriers being notoriously averse to VoIP as it threatens their business model, I don't think Skype will be integrated into Windows Phone 7. A more likely target will be to integrate it into Windows itself, most likely for tablets too. Skype runs on many operating systems already - iOS, Mac OS X, Linux, Android, Windows - so it would give them a pretty universal VoIP solution from the get-go.
Of course, assuming Microsoft is interested in properly maintaining the Linux version at all. Not that Skype itself has had much love for Linux, but at least Skype ran on Linux (somewhat), and now with Microsoft at the helm, I'm not so sure that's going to last.