We released version 4 our our API back in July of 2010. At that time we indicated that support for version 3 of the API would stop at the end of 2010. We have now picked Feb 1, 2011 as the date when V3 will be turned off. Calls made to V3 of the API will no longer work after Jan 31, 2011.
What does this mean for you? If you already using V4 of the APIs you do not need to do anything. Likewise, if you are using a recent version of Pyechonest (our Python client), Remix or jEN (our Java client), you are already using V4 and don't need to do anything. If, however, you are using V3 of our APIs, then you will need to switch to V4 by Feb 1, 2011, otherwise your application will stop working. But, really, you should already by using V4 since it is faster, has many more features and supports all sorts of coolness like Rosetta Stone, JSON, JSONP and so on.