The following images comes from the last week's WP Dev Podcast, detailing a set of features rumored to be announced on May 24 "Mango" event.
The first screenshot shows "Messaging Hub" chat integration, showing that Windows Live Messenger and Facebook Chat are built right into the platform with Windows Phone OS 7.5. It also shows an option named "threads" which possibly could be "SMS". Also shown is a "groups" which simply allow you to email or text a large group of people in one action.
Following screenshots show off the Skydrive integration to the Office Hub, allowing the user to save and retrieve documents from the cloud (Skydrive is currently "supported" but only OneNote is able to utilize the service.
Games hub has been overhauled, and will allow cloud sync of saved games and settings (similar to "Steam Sync" where games synchronize across PC's).
Finally, pictured below is a feature called "Show album art on Lockscreen." The feature is rumored to be a dynamic display, similar to that of the Zune desktop experience (rather than static album art).