This doesn't work: the XEN0 config hasn't a "acpicpu" option and if trying to add it, it refuses to compile. So for now, no power management for me on Xen Dom0 :(
Power management under Xen requires ACPI support, as the hypervisor does not implement extensive power management routines. Today's machines requires ACPI parsing to query/obtain information regarding almost everything, including PM, and this task is devoted to dom0. The support is missing for NetBSD.
There are some quirks for playing with CPUs [1], but IMHO, it does not qualify as power "management" :o
[1] http://mail-index.netbsd.org/port-xen/2010/05/22/msg006014.html -- Jean-Yves Migeon jeanyves.migeon%free.fr@localhost