NEVER ! I don't like capacitive as a screen technology very much. Enough that the hype it created (yes, I know, the iphone has it so it must be one of the keys for success) makes it being pushed on every phone. Why would I want it ruining my keyboard too.
If you like capacitive keyboards just get a phone without a keyboard with the added benefit of less weight

It could work for a luxury-line phone. You know one of those that cost a fortune and run S40 1st edition or stuff, but look really good. Put in an OS that would support this kind of touch input and let people play the worm game with it looking bling.

In all seriousness, I would bet that a manufacturer like LG would be the company coming out with something like this.

Being able to rest your fingers on it would be a deal breaker for me - I want to be able to slide my fingers across all the buttons but only press them when I actually want to press a button.

Having capacitive keyboard just for shiny-shiny sake is pointless... you'll probably ride a small wave of hype, but there's gotta be something behind it to sustain the sales/adoption rate.

Now, if that capacitive screen is meant to be used for a broader range of functions (Gaming input, multitouch apps input, etc); then that'd be pretty neat.

ps: oh lookie: http://www.youtube.com/watch?v=za8V2IiGCfY

Hit the nail on the head there I think.

I rested my fingers on the keyboard, and right now on my laptop I do. For me, it's got to be one or the other. I'm sure they can make much thinner button keyboards which have a decent feel too them.

I'd rather have e-ink backed keyboard keys. Physical keys like the N900's but with an eink background that changes depending on context.

Completely not practical but would be awesome nonetheless.