There are technical limitations to VMotion around CPU type, so for example, you may have separate clusters for Intel and AMD systems as you cannot do a live migration between the two. I generally advocate a single large cluster for most small to medium datacenter, up to the supported limit of 32 ESX hosts per cluster.
The reason you don't need separate clusters is because the resource pool feature in VI3 allows you to partition your host resources in a logical manner rather than a physical one, so it really doesn't matter which host you're running on provided you have resources available *somewhere* in your cluster.
"Be there. Aloha." -- Steve McGarret, _Hawaii Five-Oh_