As of late I have seen multiple mentions of vSEL – and always by VMware employees.
So what is this vSEL that they are talking about?
Google led me to some answers. Hany Michael (now a VMware employee) – has a comment from Mike Dipetrillo on his post

VMware also has an internal environment called vSEL that runs nearly all of the products in a nested environment. vSEL (the virtual SE Lab) let’s our tech resources in the field deploy and learn our applications as well as do demos and training with customers and partners. This “cloud” services over 1,200 tech people today inside of VMware.

From the VMworld Blog: The VMworld 2011 Demo Cloud
The best resource to learn about vSEL will be the session given by Ford Donald at VMworld 2011:
Virtual SE Lab (vSEL) Building the VMware Hybrid Cloud CIM 1436




This session is not public – unless you were at VMworld and have access to the sessions (but it will not be long until VMware allow public access to all of last year’s content). It is an interesting session and gives a lot more detail about the environment.
Update: The sessions were released - so you the link now points to the VMworldTV channel and the video is embedded above.

Unfortunately the I see that the session 2350 Evolving VMware vSEL: The Journey to OneCloud - Ford Donald is not listed on the catalog – so I am not sure it will be at VMworld – it would have been interesting to hear.
The VMware vSEL portal - https://vsel.vmware.com/
(Disclaimer – all the information here is openly accessible on the internet. I have never actually had access to this portal – and this of course is a VMware internal-only resource. I am sure that if possible – a VMware employee will be able to provide more details on exactly what the full feature set contains).