There's a note on top of the docs:
Note: The Google Web Search API has
been officially deprecated as of
November 1, 2010. It will continue to
work as per our deprecation policy,
but the number of requests you may
make per day will be limited.
Therefore, we encourage you to move to
the new Custom Search API.
The deprecation policy says that they will continue to run the API for 3 years. So if you already have an application that uses the old API, you don't have to rush to change things just yet. If you're writing a new application, use the Custom Search API. See my answer here for how to do this in Python, but the idea's the same for any language.
