Hours before Mobile World Congress officially kicks off, Sony (NYSE: SNE) Ericsson (NSDQ: ERIC) announced a new premium brand of phones today, saying that it will run Microsoft’s (NSDQ: MSFT) Windows Mobile operating system. The first phone called Xperia X1 will launch for an undisclosed price in the second half of this year and be sold worldwide, including the U.S., according to the AP.

The phone appears to not only be a work horse, but to have a rich multimedia experience. Running on HSDPA/HSUPA, it will be fast. The slider phone reveals a full QWERTY keyboard and the screen is touch sensitive. It has a 3.2 megapixel camera and something called Xperia panels which sounds awfully like widgets that allow a person to navigate nine applications with their fingertip.

This is the first time Sony Ericsson will be using Microsoft’s operating system. Typically, the company produced phones on the Symbian platform, owned partly by Sony Ericsson and primarily by Nokia (NYSE: NOK). The fact that it will be running the Windows operating system was definitely downplayed. In the press release, Sony Ericsson mentioned it briefly in the second graph, and didn’t elaborate until the end. The AP reported that executives said their focus was on the user experience, not the operating system. Despite this, Sony Ericsson executive Steve Walker said Microsoft Mobile was the “ideal” platform for the X1.

The deal is significant for Microsoft, which is seeing stiff competition from Apple’s (NSDQ: AAPL) iPhone. Now with Sony Ericsson under its belt, Microsoft will be serving all but one of the big five cellphone manufacturers—Nokia. according to the International Herald Tribune.

Coincidentally, rampant rumors last week suggested that Microsoft may also be working on a partnership with Nokia. Those appear untrue for now. Scott Rockfeld a Microsoft marketing manager for Windows Mobile said: “Nokia does license quite a bit of Microsoft technology including Windows Live, Exchange ActiveSync, and PlayReady, to name a few. However, we’re not in discussions with Nokia about licensing Windows Mobile.”

The rather-strange video ad introducing Xperia, embedded below (RSS readers will have to click through):

Also, a hand-on video review of sorts of the phone, from Slashgear/PhoneMag.com: