TL;DR; There is no 'new' Raspberry Pi expected in the near future. *
Eben Upton, Founder, has stated in public that he would expect a NEW board between two and three years from now (Sept 2013). In all likelihood,this would be a device with a new SoC with more power and more memory, and perhaps other features yet to be decided.
I (personal opinion) expect that the upgrade path to any new device will be simple, and code will be compatible between devices, to ensure smooth transitions to any new device.
In the meantime, there will continue to be new revisions of the current device (same SoC) as and when they are required and are cost effective. Again, there will be no compatibility issues between device revisions.
The Foundation are putting a lot of money in to optimising code to run on the Raspberry Pi, both at the graphics level (Weston/Wayland and others) and the apps level (Scratch and others), which will ensure the device remains competitive over its lifetime.
* New does not include B+. As stated in the Model B+ product announcement, this is the final evolution of the original Raspberry Pi design.