I too understand the enormity of upgrading a large corporation - However I also believe that for the majority of users upgrading the OS itself is not a major concern - what concerns users is change to what the use - the applications.
The average corporate user will have many more issues converting from Office 2003 to Office 2007 than from XP to Windows 7
The experience of the Beta testers for windows 7 has shown that application or hardware support is not a major issue with Windows 7 and the applications are recognisable running on 7
The people who actually interface with the OS are the more advanced user and they are probably capable of making the change
The group that are most effected by potential change are the IT departments that have to actually get the system working, and of course have they the most say as to what and when changes.
A lot of IT people I have contact with do not have a desire for change - for some XP is all they have known.
Microsoft has to persuade these people that the change is needed - not the actual users, or the Small Office/home users, (they use the OS on their PC as Shipped), or of course the enthusiasts, they've already made the change*