Microsoft usually does offer an upgrade option with a lower price than the full version.
And with release dates looming closer, deployment usually does somewhat stall before a new OS release. A lot of users decided to wait on buying a new PC or purchasing Windows XP several months before the release of Windows XP.
These are both reasons why Microsoft had offered the free upgrade (attractive upgrade pricing?) for OEM XP purchases before Vista was released. Express Upgrade, anyone? I'd welcome that with Windows 7 any time.
Microsoft just needs to deliver this time. We were promised a lot before the release, but a lot was cut out of the final release. WinFS, for example. I am still a huge fan of Vista, and haven't had any issues with it, though!

Been using it since early beta's, and loving it! Windows 7 looks to bring a lot of fixes to the Vista core, and basically be what Vista was meant to be.