Where can I download the LEGO Mindstorms NXT 2.0 IDE?
LEGO has made it possible to download a stand-alone installer for NXT-G 2.0. It's compatible with both Mac and Windows.

Alternatively, you can download a disc image of the NXT-G 2.0 CD.  The link contains instructions on how to use it (as it isn't as easy as downloading just an installer).