After weeks of rumor and speculation Mojang have conformed that they are indeed being bought by Microsoft. The confirmation comes from the Mojang website seen here. Where they put the rumors to bed.
Yes, the deal is real. Mojang is being bought by Microsoft.The post opens.
Mojang gives some reasons as to why they would sell Minecraft.
Why did you sell Minecraft?Perhaps one of the biggest things about this confirmation is that Notch and the other founders Carl and Jakob are leaving.
Minecraft has grown from a simple game to a project of monumental significance. Though we’re massively proud of what Minecraft has become, it was never Notch’s intention for it to get this big.
The founders: Notch, Carl, and Jakob are leaving. We don’t know what they’re planning. It won’t be Minecraft-related but it will probably be cool.They also confirmed the deal was in fact 2.5 Billion dollars. Some more questions about the deal are answered in the Mojang post seen here.
Update: Notch has also posted a piece called "I'm leaving Mojang" on his personal website seen here. He gives some deeper insight into his decisions.
Source: Mojang Website
Source: Notch's site