I have tested this functionality with the "stable" Debian 6.0.7 amd64 (squeeze) and Debian 7.0 amd64 (wheezy) hosts and cannot reproduce the problem: "Power on to BIOS" is available whenever the VM is powered off, and selecting it has the expected effect. I don't have a sid install available to try on. More information from mfelker might help narrow down whether it is specific to the VMs or some configuration or just the host OS. We certainly expect this feature to work correctly in the Tech Preview.
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Darius