How to put iPhone 3G into DFU mode when power button doesn't work
iClarified has provided this post that might be of use to you. It explains how to put your iPhone in DFU mode without using the power or home button.

Whatever you mean to do, just be sure to read the post completely before doing so, as without a power button you won't be able to shutdown your iPhone to reboot it again. Unless you let it go completely flat - this would take a while.