One of the biggest sticking points with ESX, in my opinion, has been the VMotion compatibility and processor classes. This point has produced some rather interesting support documents both from VMware and hardware vendors. VMware has been working to alleviate this problem and demo’d the solution during VMorld again. The solution is called “Enhanced VMotion.”
The basic concept is that it takes a group of heterogenous processors, determines the lowest common denominator between the group and then sets that as the baseline for the cluster. After that point, ESX will basically mask all newer processors down to the most compatible feature set.