If you want to run a ton of VMs on a server, Xen is great. It's fast and stable once you sort through the mess of getting a kernel that both supports your hardware and runs well as a Dom0 (the "host" machine).

And for the past year or two, most distros have shipped kernels which would boot just fine "out of the box" on Xen virtual machines.

It's what most "cloud" or VPS providers run (including Amazon, Rackspace, Slicehost, Linode, etc).

However, if you're running a desktop and want to virtualize, Xen is probably going to be a HUGE pain in the balls, with no real benefits. Just use KVM or VirtualBox.

Dustn Sallings put the code on Github and has already hacked some basic Snow Leopard support and a minimal installer:

http://dustin.github.com/2009/10/23/mac-zfs.html

Code's here, fork away:

http://github.com/dustin/mac-zfs

MailWrangler didn't get banned for being "competitive," per se. It was because it "duplicated the functionality" of MobileMail. Ditto for Podcaster duplicating the iPod app.

The legitimate, user-positive argument Apple would likely make is they want the device to be easy to use and not confusing, such that there's no compelling reason to have a million different music players or Mail apps when theirs are great.

It's misleading to describe Apple's motivations as being anti-competive. Nobody knows that they're thinking. And they seem hellbent on not explaining themselves.

It is, of course, totally and completely stupid for Apple not to address these questions directly, be more clear about what will or won't be allowed, and to allow both MailWrangler and Podcaster on the store. It's just dumb, dumb, dumb.