I just finished reading a good post on Wikibon by the "other" Scott Lowe where he discusses the differences between vSphere and Hyper-V and what the adoption rate of Hyper-V will likely be. He delves into the cost comparisons being made by both VMware and Microsoft as well as broadly touching on the differences of their management approaches. I encourage people to read his article, it's not that long, but the take away is that he thinks Hyper-V is catching up to vSphere in functionality, that vSphere is better suited for enterprise installations and that pricing pressure will impact the margins of both companies.
Smart CIOs and IT leaders will try to find ways to run both hypervisors in their data centers. For starters, the only way to future proof your data center from vendor lock-in is to have skills in competing technologies that can replace each other. Moreover, shrewd negotiators know the most powerful word in their vocabulary is "no". But no doesn't mean no if you can't actually implement the changes you decide to make and that takes a commitment and that has costs associated with it.
Freedom from lock-in isn't free, neither is negotiating leverage. It's prudent to avoid painting your organization into a corner and having to deal with being stuck in a place where the only way out is long and expensive.