IceCream Sandwich will be brining many goodies however folks at AndroidCentral have been playing around with ICS on Nexus S when they had hit the keyboard settings were they found theme support natively present. There are six native themes already built in to the ICS although they are not very impressing but the support it self open a new horizon to Android keyboard themeing. Also the themes will be downloadable from Android Market as other applications.
Themes support in complete Android interface is yet not available as present in Apple iPhone, iPad and iPod Touch using Winterboard but with the addition of Keyboard theme support Android is also planning to progress towards the aim of complete interface revamp. Although currently Android Widgets and Plugins to some extent change the interface however complete redesigns are still far away.
Keeping all these new features in view and overall performance improvement the upgrade to IceCream Sandwich is a sure GO and its expected to be released in a weeks time. SDK for Android 4.0 alongwith all the official features of Android IceCream Sandwich can be viewed at Android Developer Portal.