I'm sure VMware has been studying netscape vs microsoft with a very good spyglass and they've decided to let this trial balloon go up to see if they can harness the oss community to help them out.
It's very much appreciated that they did this, let's hope that they will do more!
This is not in anyway a trial with open source software. FOSS developers work on software that they're familiar with. I doubt very many of them own thin client hardware. They also don't like proprietary software and on the other end of this client is VMWare's closed-source server software.
VMWare is open sourcing this product because thin clients tend to run on exotic hardware and they want to allow manufacturers to easily port and install their client. That ensures that they'll be locked into VMWare's server product.
It's very much appreciated that they did this, let's hope that they will do more!