I tested 2.6.21-rc1 on my laptop (IBM X20 with 440BX) and found two problems:please take a look at 8066 bug and patches there.
First, a seemingly harmless one - ACPI error messages during bootup:
ACPI Exception (evregion-0420): AE_NOT_FOUND, Returned by Handler for [EmbeddedControl] [20070126]
ACPI Exception (dswexec-0462): AE_NOT_FOUND, While resolving operands for [OpcodeName unavailable] [20070126]
ACPI Error (psparse-0537): Method parse/execution failed [\_SB_.PCI0.ISA_.EC__.BAT0._STA] (Node d3c435e0), AE_NOT_FOUND
ACPI Exception (evregion-0420): AE_NOT_FOUND, Returned by Handler for [EmbeddedControl] [20070126]
ACPI Error (psparse-0537): Method parse/execution failed [\_SB_.PCI0.ISA_.EC__.AC__._PSR] (Node d3c43540), AE_NOT_FOUND
ACPI Exception (ac-0095): AE_NOT_FOUND, Error reading AC Adapter state [20070126]
And second, there were ACPI error messages about EC__ during shutdown. These messages appeared, then the "acpi_power_off called" message appeared, then it waited for about 5 seconds (this does not happen normally) and then it succeeded shutting down. Captured a screenshot of these messages, temporarily available at http://www.cs.ut.ee/~mroos/x20-shutdown-acpi-errors.jpg