Microsoft CEO Steve Ballmer said the next Windows Phone OS update codenamed Mango will bring more than 500 new features. Speaking at a developers' forum in Japan on Monday, Ballmer didn't give away too many details about the update but promised to unveil more on Tuesday at a scheduled event in New York. Until then, we have a clue or two about what's coming.

Ahead of a scheduled preview of Windows Phone 7 Mango in New York, Ballmer also teased about an announcement regarding several new devices. The WMPowerUser blog says it has received a tip saying that we can expect up to nine new WP7 devices from HTC, Acer, Dell and Samsung-but they won't be on sale until the holiday season.

What's coming up in Mango (Windows Phone 7.5)

Microsoft pre-announced several much-needed improvements to Windows Phone 7 to keep people's minds off some troubles with software updates earlier this year. Third-party app multitasking is probably one of the biggest drawbacks in WP7, and Mango should fix that.

Also coming up in Mango is a new version of Internet Explorer 9 Mobile, support for Office 365 and SkyDrive in the Office Hub, Twitter integration in the People Hub, a revamped Games Hub and custom ringtones support. Mango will also allow users to configure services running in the background and bring pinnable email folders and conversation view.

There are also a bunch of features Microsoft didn't announce but were leaked in a developers' podcast last week. These include Windows Live Messenger, Facebook Chat and AIM integration in the Messaging Hub, as well as group messaging. Turn-by-turn navigation is also on the table, along with SMS dictation and iPhone-like visual voicemail.

The Windows Phone 7 Mango update is expected to arrive later this year.