Microsoft's Windows Phone Developer Conference has been trimmed from a two-day to a single-day event. Registration for the June 20 Windows Phone Summit is open as of June 4.

In early May, Microsoft announced plans for a two-day Windows Phone Developer Summit. At that time, Microsoft said more information on registration for the conference would be available "in the coming weeks." A number of developers have been asking me in the past week or two if the conference was cancelled, since there was no additional information available.

Interestingly, today's updated invitation doesn't call the one-day event a developer summit; it's simply labeled as a "sneak peek" of the future of Windows Phone. But it is still definitely for developers.

According to Microsoft, "invited developers" can complete the registration process at https://www.ustechsregister.com/WPdevsummit/RegistrationSelect.aspx. Registration is limited, but the event will be open to everyone via a webcast on Microsoft's Channel 9 at http://aka.ms/wpsummit.

Microsoft officials haven't said publicly what will be on the Windows Phone Summit agenda. But given the timing and tips from various sources, it seems that the Windows Phone 8 operating system, codenamed Apollo, will likely be the star of the show. The latest invite says the conference will be about "the future of Windows Phone," making the Apollo focus a strong likelihood.

Earlier this year, there were some major leaks as to what Microsoft was planning to deliver as part of the feature set for the next version of the Windows Phone OS. Among the planned features:

* Support for multicore processors * Support for four new screen resolutions * Support for removable microSD card storage * Support for NFC and an associated “Wallet Experience” * Inclusion of core Windows elements, including kernel, networking stacks, security, and multimedia support * New data-tracking capabilities, showing users a breakdown of their data consumption by various networks * Use of a proxy server to deliver pages more efficiently and quickly to Internet Explorer 10 Mobile * Addition of native BitLocker encryption and Secure Boot * A separate but improved Skype application, but not integration of Skype into the operating system * Replacement of the Zune PC client software with an update mechanism more akin to ActiveSync. (The new update mechanism is codenamed Daphne, according to one of my contacts.)

Microsoft officials have continued to decline to say whether existing Windows Phones will be able to run the Windows Phone 8 operating system. My contacts said earlier this year that this would not be the case, as did unnamed sources speaking to The Verge.

Microsoft also has shared previously a few tidbits about its Windows Phone OS 8 developer strategy. Microsoft is believed to be looking to unify its PC and phone developer ecosystem, and is expected to alter its current Windows Phone toolset and guidance to mirror that offered for Metro-Style apps for Windows 8.

Microsoft officials have said that existing Windows Phone apps will run on Windows Phone 8 devices, and that Microsoft will continue to support XNA to some degree with Windows Phone 8. They’ve been vaguer about plans for Silverlight support for the Windows Phone 8 platform.

Last month, Nokia -- Microsoft's closest Windows Phone OEM partner -- announced it was nixing its planned Nokia World 2012 conference, which had been slated for Helsinki in late September. Instead, Nokia will hold a number of smaller, more focused conferences for specific audiences, starting with an invitation-only event in early September for its operator and retail partners.