Although I currently use Linux and ESXi as hosts, I would prefer to see the FreeBSD operating supported not only as a host, but as the basis for a future version of ESXi. It's without a doubt: FreeBSD provides better overall performance when compared to Linux, and (maybe) even Windows / OS X, especially under heavy I/O load.
I really hate that Linux "locks up" when the I/O load gets high. This has never been a problem in Windows, Mac OS X, or FreeBSD system no matter how much I/O load I put on it. Not even the new scheduler or the "amazing patch" helps. In a Linux virtual machine host, all it takes is one virtual machine with heavy disk usage to bring down the whole system. It would be great to have FreeBSD as an option instead of Windows and OS X to avoid such a problem.
Hopefully VMware is working on something. I really think FreeBSD deserves more attention than it's getting in general. It's by far a better designed operating system, and FreeBSD as a VMWare host would be a perfect match. It sure would solve the constant isssues of having to patch and rebuild the kernel modules every time a new Linux kernel is released. FreeBSD doesn't have that problem.