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Not all virualisation solutions are equal (Score:2)
I've been a heavy user of Linux vservers (see also: OpenVZ and the other one whose name I forget; and BSD 'jails') for a number of years. They are similar in concept to your chroot solution except much more convenient to use. They are lightweight because the virtual servers don't run their own kernels - they share the host kernel but with key structures like the process table partitioned into separate namespaces.
I've found this type of virtualisation to be invaluable in the 'staging' or 'User Acceptance
Deduplication might help with some of this problem (Score:2)
One thing we see coming up in our giant work VM system is deduplication, where the VM software natively lets you copy an entire VM machine with nearly zero disk cost, and can reuse the same disk for multiple live VMs.
I am really hoping this filters down to commonly available VM software soon.