Original bug reported on launchpad: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/510571
Kernels past a specific version won't boot along with acpi in virtualbox
Boot hangs at either "Checking 'hlt' instruction... OK."
OR "SMP alternatives: switching to UP code"
(at which point virtualbox uses close to 100% cpu also)
Booting with the acpi=off flag, or disabling acpi in virtualbox allows normal boot.
version 2.6.32-10.14 (ubuntu)
based on 126.96.36.199, works fine
version 2.6.32-11-15 (ubuntu)
based on 188.8.131.52
is the first to exhibit the issue
along with later
version 2.6.33-999.201001191335 (ubuntu mainline daily)
based on long<v2.6.33-rc4-399-g24bc734>
version 2.6.33-999.201002061003 (ubuntu mainline daily)
based on: long<v2.6.33-rc6-228-g56dca4c>
...which fails similarly.
According to comments on the launchpad bug, this seems to be a problem in virtualbox rather. Closing.