"ESX 3.5 was built on top of RHEL" again far from accurate. The COS has been based on RHEL since the genesis of ESX. Whereas VMkernel is VMware's own creation. At least thats what they claim.
Even if the COS in ESX 4.X is based on SLES instead of RHEL, your point does not still make sense because VMware's future roadmap is to phase out Classic ESX. Sometime in the future, there will be only ESXi.
This is my first comment here. Oh Boy! how easy and fast it was to sign up!!
I sign just to comment because the comment "(VSphere, 4i) are built on top of the SuSE Linux kernel" is so wrong. The Classic ESX has Service Console is is customized RHEL. ESXi does not have the Service Console. It still does have a command console where one can run ESX commands and a limited set of Unix commands which AFAIK are Busybox's. It also has dropbear sshd.
So coming back to the topic, I too have no idea what VMware is going to do with SuSe Linux.