According to this source, there are currently two million Xbox units left, with 400,000 of those dedicated to the European market. Once those are gone, there will not be any more made.

However, Microsoft has indicated to key retailers that it plans to having around a million Xbox 360 hardware units available in time for Christmas of 2005…

Further proof that the release of the XBox 360/Next/whatever is close at hand. Also proof that current XBox titles will be compatible with the next generation Xbox.