Plesk 11.0 previews are available at http://www.parallels.com/download/plesk/11 and will be updated weekly.
You can download a build and a license key there.
The recommended evaluation path would be either to restore backup of live machine there or clone a live machine and attempt upgrade. In case there would be a problem, either you would notice it as a result of this operations OR it will be noticed by Parallels (in case you didn't choose to block sending problem reports).
Changelog is available (but yet more items to be delivered in next builds). Things to pay special attention at:
- cloning feature makes preparing and applying images for hypervisor VMs (Xen, PSBM, KVM, etc) much easier