As part of the company's effort to move their products to a unified experience, Microsoft has started to roll out a metro-styled Bing to users of the search service. The new look incorporates the Live Tiles and Metro design rules that Microsoft now uses on many of its other products.
First spotted by WithinWindows, the new look is slowly being rolled out to the Bing homepage in waves, similar to the rollout methods that Live Mail uses. Right now, it looks to be US only, but it's expected to roll out worldwide over time.
We started to see the start of Microsoft’s unification strategy when they released Windows Phone 7, which took the metro UI that was first seen in Windows Media Center to a new level. The company has been deploying this design mentality across many of their products including the Xbox platform and its upcoming Windows 8 release.
Image Credit: WithinWindows.com
[Update] As pointed out by @robertmclaws, this could simply be a test. Until we hear official word from Microsoft, take it with a grain of salt that not everyone may get this new look for some time.