If the button has failed altogether, you can also use [http://jaxov.com/2010/05/recboot-iphone-recovery-mode/|RecBoot] to put your phone in recovery mode. Like Brad says, while in recovery mode, if you unplug it, it will turn off after a while. To turn it back on, plug back in and use RecBoot again to bring it out of recovery mode.
Not exactly an everyday solution, but should work if you want to take it apart without having to run the battery down completely. :)