I can not reproduce this on the current natty kernel (2.6.35-1101.4).
On maverick with the latest updated kernel (2.6.35-903.21) running this command shuts down the cpu with no dmesg error, however the system doesn't return to a shell prompt. Opening a new terminal and running with 1 reenables the cpu, but also fails to return the shell prompt.
I can not reproduce this on the current natty kernel (2.6.35-1101.4).
On maverick with the latest updated kernel (2.6.35-903.21) running this command shuts down the cpu with no dmesg error, however the system doesn't return to a shell prompt. Opening a new terminal and running with 1 reenables the cpu, but also fails to return the shell prompt.