It is probably very well documented - it is in Microsoft's best interest that people upgrade to 2003 Server and XP Pro. However many do not see what benefits they offer over what they already have (i.e. Windows 2000). NT4 to XP or Server 2003 is a lot
more appealing than upgrading from 2000.
I know updating computer systems is needed, but the price and time/manpower can be prohibitive (money spent on upgrading PII's to P4's just to run XP could be spent somewhere else, especially when all those PII's do is a bit of Word Processing). Also a lot
of the viruses are targeted at 2000/XP now, so Windows 98 is actually safer from infection (still insecure though).
It may take yearsjust to upgrade people to XP - and once that happens, Microsoft will want people to upgrade to Longhorn (and people may be forced to eventually when 2000/XP are no longer supported).