1) Microsoft's Hyper-V will be at 1.0 when it arrives in late summer-- if then. VMWare's doing great things, including building partnership communities similar to those in the Xen world, *now*.
2) No doubt VMWare will adjust prices at some point. They can't keep going the way that they have in the face of nearly free offerings (by comparison) from others.
3) Marketshare and mindshare are important, and VMWare has momentum. 80% is tough to beat.
4) The miscellaneous forks of Xen don't help it much. There's Citrix/XenSource, the Xen Project itself, xVM, Virtual Iron, and the ostensible but unproven Hyper-V components. This is a disarray that IT people are unlikely to tolerate in what amounts to a mandatory HA environment.

Overall, VMWare's going to need flexibility to retain marketshare. Hyper-V is so much gas until it's production, and then we'll wait at least several quarters before wanting to put it into production. Xen is very important, and it's not up to the quality of VMWare in many ways, including cohesiveness, 3rd party support, drivers/compatibility, and 'community'. Maybe it'll catch up. Maybe not.