When you install Mac OS X Lion in the VM, you can use the option to copy an existing Mac OS X Lion environment from a Time Machine backup, or you can use the Migration Assistant utility to transfer another environment's settings and apps to the VM. But you can see such external environments only if they are on USB-connected storage devices and you've told Parallels to assign that USB device to the Mac VM, which you do via its Devices menu's USB submenu. (Note that the name of the USB drive will not likely match the name of the disk partitions on that drive.)
Sadly, Parallels does not see storage devices connected by the speedier FireWire technology or by the even faster Thunderbolt technology -- the kinds of technologies that Web and application developers would want to use when running Mac OS X from the external disks from which they'd likely be migrating Mac OS X environments. Most such drives -- but not all -- have USB ports, so you can connect them via USB for the VM setup and then use FireWire or Thunderbolt from that point on. Just note that non-USB drives will be visible within the VM only as folders, so any apps that need to see them as drives, such as Disk Utility or Time Machine, will not be able to connect to them.
Finally, I could not install a beta version of Mac OS X Lion, which I've been using to test iCloud as a registered Mac developer, over a VM installation of OS X Lion; the boot screen hangs up. The developers at Parallels said they hadn't tested the installation of beta Mac OS X versions in a VM, but were surprised I was having trouble. This may reflect a bug in Parallels Desktop or an issue with this particular version of the beta Mac OS X installer. I'll update this review if we're able to resolve the issue.
These installation limitations mean that Parallels Desktop 7 is not nearly as flexible as what Mac OS X lets you do so easily with physical disks. After all, portable hard drives are cheap -- and quite flexible to use for having multiple Mac OS X instances.
The shortcomings suggest that Parallels Desktop's installation method for Mac OS X VMs was rushed to get a product out the door quickly after Lion's release -- after talking to the developers at Parallels, it's clear they know about the various ways to install Mac OS X. Whatever the cause, it makes Parallels Desktop 7's appeal more limited than it could have been, especially for the very developers and power users most likely to be interested in Desktop 7's new Mac VM capability.
If you're not afraid to get your hands dirty, you can work around these issues. I just hope Parallels comes out with an update soon that doesn't require diving into the nitty-gritty. And that adds FireWire support.