So the question posed was should VMware open source ESX Server?  Edward Aractingi, of Edward's Blog, sure thinks so.  While I doubt this will happen, at least any time soon (CYA), Edward lists a few arguments for his case:

• ESX is already using many open source technologies and components, whether we say it's built on top of Linux OS, using Linux kernel, or is a Linux derivative, it is built on an open source foundation.
  
• ESX Server make Linux users feel at home, and Linux community are used to find, evaluate and use open source projects before they recommend it to their employers and use it in the enterprise, imagine a project called OpenESX, and fully open source and free virutualization project where anyone if the world can contribute to the design, development, testing, and documenting of every part of it just like Fedora project and Redhat. VMware will sponsor the project and support the development.
  
• It will provide rapid progress, more innovation, closeness to the users, and larger user exposure: open source's advantages.
  
• It is Microsoft Virtual Server's competitor, many Open Source contributors build their environments on open source platforms and never user Microsoft's software unless they have to, those users and developers will definitely contribute to an open source project that will integrate with their current network, provide enterprise class support, without migrating to Windows hosts or Microsoft technologies.
  
• By saying open source, and I'm not saying ESX will be totaly free and VMware going out of business, I admire VMware innovation and I actually think VMware business will be in a good shape with Open Source community support, (I don't want to go through the debate of opening Microsoft Windows source now, but I think the open source business model will be suitable for VMware)
  
• It is the best way to [defeat] Open Source rivals, Xen, Qemu and other open source alternatives' advantage is user contribution and rapid progress, Vmware will soon realize that once those products are mature, it will be competing with them in the enterprise market along with the pressure of Microsoft.
  
• VMware has always supported and relied on user community, VMTN is a big strength for VMware because of the users forums, blogs, local groups and add-ons, and they can give those users access to the core technologies where they can suggest, enhance and innovate to the seek of virtualization users back to VMware clients.
  
• VMware already experience how much users base was expanded by giving away Vmware Server for free, those users most likely are going to recommend VMware as solution provider for virutalization after evaluating the product, and ESX will have much more powerful but similar affect.

Now if you are a non-believer like me, he does end with a good point.  Just think about Java and Solaris... after 10 years, Sun decided to open source those to stay in the competition.  However, I would argue that at the moment and for the foreseeable future, VMware's competition has a long way to catch up, unlike what Sun had to combat.  VMware currently controls around 80% of the market according to many industry surveys.  Let's take a look at them in 2009, after 10 years, and see where they are.

Nice list Edward.  You can view Edward's blog, here.