I love capacitive buttons. Just not on my phone.
The new XB360 Slim, for example, has them for the power/eject buttons and I love it. Looks slick. No buttons to push. Very quick to use. Love it!
As for on my phones they're okay, but for an iPhone I wouldn't care for it. I'd press it accidentally, etc, because of its central location for it's main button. I would prefer to have a physical button on that particular design.
Maybe a capacitive screen on/off on the side of the phone, but that may be too odd looking w/o all buttons being flush.
** NOTE: Right now I have a Motorola Droid so it's got the 4 main buttons in which I'm referencing on "my phone" as of right now.