Nick, the problem with excel's tabs is not the ability to navigate, it's the
visibility of them.
Most of the time excel is used as a reporting tool - and the results are
sent from one user to another.
Unfortunately, the receiving user has to know that there are additional tabs
that need to be viewed.

Ideally, excel should have the ability to include a frontis piece - a
summary of the contents of the workbook, with links to each sheet embedded.  
The frontis piece should be the first item displayed to the receiving user -
not the data on the first sheet.  The creation of the frontis piece should be
user selected, but wizard/template driven.

Obviously, this is only necessary when you send a workbook to another user
as a report - not when you are creating a sheet to do some calcs.