The virtualization story for Mac OS X is about to change dramatically, and for the better, as Lion's licensing changes the rules for virtual machines.
For some enterprise deployments, virtual Mac OS X environments are the Holy Grail: giving access to Mac-only applications on demand without having to supply Mac hardware on a one-to-one basis. While the vanilla version of Mac OS X has been theoretically virtualizable since the Intel transition (and in fact can be run on a virtual machine now under the right circumstances), the licensing agreement for Mac users up until Mac OS X 10.5 Leopard didn't allow virtual Macs, full stop.
Starting with Leopard, Apple began to permit limited virtualization of Mac OS X, with two major caveats: you could only run VMs on Mac hardware (no blade server racks full of HP gear serving out Mac desktops), and you needed a Mac OS X Server license, with a steep price. Under these conditions, virtual Macs were a luxury few took advantage of.
Now Lion's new EULA is set to change all that, as reported by MacRumors. 10.7 users will be permitted to run one or two virtual Mac instances on each physical Mac, presumably using existing virtualization tools like VMware Fusion, Parallels, VirtualBox or others. This is bound to be a big help for developers, IT managers and others who need to keep a known-good test environment or try out new apps in a controlled fashion.
Note that virtualized Macs aren't the same thing as virtual desktops, which Lion is also slated to support; that second feature means that you can remotely connect to your user account and your desktop 'underneath' a user who is currently logged in to the machine.
A similar capability was baked into Snow Leopard, but it required some hairy workarounds to use effectively; You can get a similar capability from the $79 iRAPP utility or the free Vine Server, but the Lion version will be single-click friendly.