Extract boot.img from an android phone

(I did this on my rooted, boot-unlocked nexus one (HTC Passion) running cyanogen cm6. Cyanogen is probably not necessary, but boot-unlocking and rooting are).

This tutorial pointed me in the right direction.

I did:

$ adb pull /dev/mtd/mtd2 boot.img-from-phone

Then put it right back using:

$ adb reboot bootloader
[wait for bootloader to come up]
$ sudo ./fastboot flash boot boot.img-from-phone
$ sudo ./fastboot reboot

I could have also taken it apart and replaced the kernel with my own.