At a jam-packed press conference at the Mobile World Congress trade show in Barcelona, Spain, Microsoft finally announced its next generation Windows Phones 7 platform, which breaks with the past in key ways while retaining the best parts of the Windows Mobile ecosystem. As expected, the Windows Phones 7 UI takes its cue from Microsoft's digital media player, the Zune. But it also goes much further than the Zune UI, offering up what Microsoft calls a delightful and fun user experience.

"Today, I'm proud to introduce Windows Phone 7 Series, the next generation of Windows Phones," said Microsoft CEO Steve Ballmer. "In a crowded market filled with phones that look the same and do the same things, I challenged the team to deliver a different kind of mobile experience. Windows Phone 7 Series marks a turning point toward phones that truly reflect the speed of people's lives and their need to connect to other people and all kinds of seamless experiences."

Windows Phones 7 Series features a home screen built from live tiles that display real-time information and spin when touched. These tiles break with the static home screens found on other devices, especially the iPhone, and allow users to get things done and see information without diving into individual applications. They're also fully customizable, providing a fully customized experience.

Devices built for Windows Phones 7 will all feature capacitive multi-touch displays (just like the iPhone) and three hardware buttons: Start, Search, and Back. (Take that, iPhone: A Back button is still sorely lacking on Apple's otherwise excellent device.) Microsoft says that the Search button is "a dedicated hardware button for Bing," providing instant access to the service no matter what the user is doing.

And if you're looking for integrated services, Windows Phones 7 has you covered: The system offers up to six "hubs," many of which appear to be based on similar user experiences in Windows Live and other Microsoft properties: People, Pictures, Games (which "delivers the first and only official Xbox LIVE experience on a phone, including Xbox LIVE games ... and avatars"), Music + Video, Marketplace, and Office.

With the new UI, one thing will remain consistent with the past: Microsoft is working with a large group of hardware and wireless partners, which will deliver new Windows Phone 7 Series devices to market by "Holiday 2010." Wireless carrier partners include AT&T, Deutsche Telekom AG, Orange, SFR, Sprint, Telecom Italia, Telef?nica, Telstra, T-Mobile USA, Verizon Wireless, and Vodafone. And Microsoft is partnering with Dell, Garmin-Asus, HTC, HP, LG, Samsung, Sony Ericsson, Toshiba, and Qualcomm to deliver devices.

More information can be found on Microsoft's new Windows Phone 7 Series website, and I'll be previewing the system this week on the SuperSite for Windows as well. Stay tuned