If you purchase new Nehalem processors in the newest HP and Dell servers (HP G6 or Dell r610,710, etc) for use in an existing VMware cluster, you will need to make sure to enable a feature called VMware EVC to enable compatibility between previous generation processors and the latest Intel processors.  Here are some links that have some very useful information about this. If you need any assistance or have questions, just ask!

http://kb.vmware.com/selfservice/microsites/microsite.do?cmd=displayKC&docType=kc&externalId=1993&sliceId=1&docTypeID=DT_KB_1_1

http://kb.vmware.com/selfservice/microsites/microsite.do?cmd=displayKC&docType=kc&externalId=1991&sliceId=1&docTypeID=DT_KB_1_1

http://kb.vmware.com/selfservice/microsites/search.do?cmd=displayKC&docType=kc&externalId=1003212&sliceId=1&docTypeID=DT_KB_1_1&dialogID=32089520&stateId=0 0 32091469

http://download.intel.com/it/pdf/VirtualizationXeon5500_final.pdf

http://blog.scottlowe.org/2009/06/10/xeon-5500-cpus-and-evc-in-vsphere/

http://www.vmware.com/files/pdf/vmotion_info_guide.pdf