I unfortunately missed the 2008 IORG conference, but it sounded really interesting.  Here are some stats offered by Jonathan Spira in his keynote:

• The cost of unnecessary interruptions plus recovery time (time spent getting back to where you were, if indeed you do get back there) to the U.S. economy is $650 billion as of 2007.
• Most interruptions are neither urgent nor important (but we think they are as we go and interrupt people anyway).
• The above represents 28% of the knowledge worker’s day.A mere 12% of the knowledge worker’s day is spent in thought or reflection.
• We spend 15% of the day searching for things and 20% in meetings.

But, is this really information overload?  Email and meeting overload, definitely, but these stats reflect knowledge worker's daily tasks, not necessarily their consumption and management of information.  In some ways, these stats reflect the connection between productivity and email overload, just as serious a problem.

Information Overload, in my mind, is a peer, rather than superset, of email overload.  Tools for dealing with information overload address searching and displaying data, while tools for dealing with email overload address bad emailing habits, and email efficiency.  Email overload is connected to productivity, while information overload is connected to executive.