so long as the servers are on the vSphere HCL when it is released then the answer is yes (note vSphere requires 64bit cpus)
on the vmotion question then if the CPU's are from the same compatibility group then the answer is also yes. As of ESX 3.5U2 using enhanced vmotion compatibility (EVC) you can create clusters containing different CPU models/generations (from same vendor can be intel OR amd but not mixed in same cluster where you want vmotion to work across ALL hosts in the cluster) and still VMotion VM's around. If your new to EVC then for VI3 environments start with this document p6 onwards
For a list of processors that work with EVC for VI3 see HERE
For general information on VMotion see the VMotion Compatibility Info Guide
For VMotion and Compatibility FAQ, see VMotion and Compatibility FAQ
For detecting and using CPU features in applications see KB Article 1005763
In vSphere there are a number of enhancements to EVC including move EVC baselines. In terms of load balancing across a cluster there are obviously advantages if all host in the cluster are of the same type/model/configuration but then there are also ways in which you can isolate certain workloads to certain hosts using affinity rules if you decide you need to keep certain VM's pinned to your new nehalem kit (you can also just disable DRS for those VM's as well of course).
hope that helps,
Lee Dilworth
