You are combining two things: (1) diskless booting which is common in HiPC clusters, where tools like compilers, libraries, etc, are stored on a clustered file system like Lustre or NFS; (2) virtual disk images (vmdk or vdi, or whatever else).
In case (1), you need a tftp server and a dhcp server, along with a modified initrd image for customized drivers, scripts, etc.
http://www.pixelchaos.net/2009/02/15/diskless-booting-with-pxe-and-nfs/
In case (2), given all machines are same, including configurations, you can nfs mount the virtual disk image. Write a script that checks for any changes like whether the existing image is same as nfs mounted; if they are not same, copy this image to boot partition and reboot it. You need to add this script in initrd. You can play with that.
http://www-conf.slac.stanford.edu/afsbestpractices/Slides/afsboot.pdf
The paper above gives you some clues. You better focus on how the whole boot process works, etc.
If you are using ESXi, there is a better way to achieve what you want: export nfs to esxi hosts. Keep the vmdk and -flat.vmdk files on the nfs server, and change vmdk location in .vmx file. During the next boot, it picks up your new image.
Best Answer
Personally I'd offer it in Virtualbox and VMWare formats, since Virtualbox is free and VMWare's player is free and well supported and popular.
What do you mean by best way to do it? If you mean offering it to users, you can always create a CD/DVD and offer it free by mail and/or offer it through a CDN or personal website; you don't mention what resources you have available to you. Do you have a website? Is this part of a business? Is it a side hobby?
How big are the resulting images going to be? Do they compress down at all?
Is this likely to be a popular download that can strain your site or tax any transfer limits you have with your provider?
Depending on how savvy your target audience is, you could also offer it via bittorrent to help alleviate the strain on your network (or if you can make agreements with other sites you can divvy it out to them as seed sites). You could even have the images hosted off-site to companies that specialize in hosting large files, so you don't need to worry about content delivery of the large files.
It kind of comes down to a question of what resources you available to you and how savvy your end users are going to be as to how elaborate the requirements you'll impose on the end user just to try out your product. Some may not like having to download separate products to try out your VM. Other sysadmins probably already have VMWare or VMWare player or Virtualbox installed.