The next iteration of Windows Phone will be called Windows Phone 7.5. So says a Microsoft Partner page anyway, which you can see a screenshot of here. "Preview the New Windows Phone OS 7.5" reads the website blurb, promising that the update will appear "later in 2011."
This is believed to be the Mango update that Microsoft has codenamed and shown off only a few weeks ago, highlighting these changes which will be available to developers from May when the dev platform becomes available:
• Application multitasking for background processing, audio and file transfer, and fast application switching • Deeper phone integration so apps can, for example, take better advantage of the versatility of Live Tiles • Access to the camera and Motion Sensor library so developers can build apps that incorporate device hardware and build augmented reality experiences