There are a lot of folks out there who have SQL Server clusters setup under Hyper-V and VMware. One of the issues on the VMware side of things has been the MPIO configuration. The reason for this problem is that VMware requires that you use what's called a fixed path to route the data between the physical server and the storage array when you are using Raw Device Mappings or RDMs.
Because you are using a fixed path if the path gets slow because a port or switch on the SAN fabric (AKA the SAN network) gets busy, there's nothing that the server can do about it. It isn't able to switch to another path to send that data down until the path that it is currently using totally fails. The other big problem that people see with having there VMs be setup on a fixed path is that the HBA in the server can fill to capacity and there's nothing that can be done about it.
Startng with VMware 5.5 this totally changes. Wih VMware 5.5 we now have the ability to use a Round-Robin path system, which is basically what you would be using with a physical SQL Server cluster. Proably the biggest benefit that most people will see is the ability for their virtual SQL Server to be able to spread their workload across multiple HBAs within the VMware host. Personlly, I'm really looking forward to this feature making it into production, and I'm sure a lot of you are as well (or at least your sysadmin teams are).