I just got a Windows phone and I'm seriously looking forward to this. The release is scheduled for fall (pretty vague huh?) and is claiming to include hundreds of updates. Click the link for a full list of features to be included so far.
Mango, which will be available this fall, includes hundreds of new features and improvements organized around three key themes: easier communications, smarter apps, and the Internet.
It doesn't appear to have a release date scheduled yet, but at least some of the "Mango" features have been talked about. Here's a quick list of what's to come so far in the new update:
Office Integration with SkyDrive and Office 365
Pinning Email Folders to the Start Screen
Conversation View in Email
Email Server Search
Complex password support
Information Rights Management support
Support for access to hidden corporate Wi-Fi networks
Facebook Chat/AIM/Live Messanger support
Improved Xbox Live game data sync
Lockscreen option to show album art of playing audio tracks
Multitasking (webOS style)
Internet Explorer Mobile 9