I'm seeing this on an OptiPlex 990 as well. Dell just released BIOS revision A03 a few days ago, and I've installed that.
Here's what I've found:
Lucid's 2.6.32-28 kernel is able to reboot the machine just fine.
Neither Maverick's 2.6.35-22 nor Natty's 2.6.38-8 kernel can reboot the machine with default options. Tried adding reboot=bios and reboot=acpi, neither helps.
I'm seeing this on an OptiPlex 990 as well. Dell just released BIOS revision A03 a few days ago, and I've installed that.
Here's what I've found:
Lucid's 2.6.32-28 kernel is able to reboot the machine just fine.
Neither Maverick's 2.6.35-22 nor Natty's 2.6.38-8 kernel can reboot the machine with default options. Tried adding reboot=bios and reboot=acpi, neither helps.
However, adding reboot=pci *does* work.