Developers, start your engines. Fresh Ice Cream Sandwich versions of the SDK and ADT have been released. There is a ton of new stuff to learn and play with. For starters, take a look at the 4.0 platform page here, download the 4.0 SDK here, and instructions for ADT 14 are over here.
I think I could write a book on all the new things in ICS, but here are the highlights for developers:
Unified UI framework for phones, tablets, and more - All the UI elements from Honeycomb, like the action bar and fragments.