Let's wait and see.
From experience currently still being made, I believe Citrix have something going that may spike both Microsoft's and VMware's wheels.
A company I am doing IT consulting for (and with) has recently dropped HyperV (after it was impossible to get it to work stably) and then decided against VMware server because Xen simply is cheaper ($ 0.00) and handles better. XenServer is a lean and mean machine that is a joy to work with -- during testing, the other two players experienced a huge variety of problems, whereas XenServer was stoically stable from installation onwards.
With their traditional close ties to MS they manage to get more varieties of Windows running stable as VMs than MS themselves do. If they can keep that up for the next few years, we may see a new market leader.