Coming on the heels of their big Surface tablet announcement this week, Microsoft is holding another event today. This time, the company is discussing Windows Phones, and is expected to unveil their next mobile operating system. Codenamed Apollo, the OS will be Microsoft’s smartphone tie-in with its new Windows 8 ecosystem.

The presentation will be part of the Windows Phone Developer Summit in San Francisco this week. Microsoft will be streaming the event starting at noon EDT, and WebProNews will be covering the event.

Exactly what Microsoft will announce is unknown, but several things are expected. The rumor most likely to be true is that the new OS will heavily resemble Windows 8 and operate on with the Metro interface. ZDNet is reporting rumors that new Windows Phones will have support for smartphones with multiple processors, higher screen resolutions, and NFC. From as far back as February there have been rumors that HTC will debut phones alongside Apollo. There is also an outside chance that Microsoft will reveal Xbox Music, which the company mentioned during its E3 press conference.

It will be interesting to see if Microsoft can keep up the momentum it has buit with the announcement of its Surface tablets. The tablets were generally well-received by the press, and their hybrid tablet/PC form factor seems to finally make sense of Windows 8. The higher-end Surface is set to compete with Apple’s MacBook Air and other Ultrabooks.