I think a unique Xbox phone would be successful, for several reasons:

Sony had a major disadvantage in trying to bring their phone to market in that a) they didn't have a suite of games that were readily available for it (whereas an Xbox phone could directly map XBLive games)

Sony didn't have a dedicated phone OS -- it had android (which was great but not special) and Playstation (which sucked, but was exclusive). MS has the Windows phone OS and the Xbox OS. Both of them are solely owned by MS, and have congruent UIs already. Mapping an XBox phone into this ecosystem would make sense and be successful.

MS could build limited kinect functionality directly into the phone. While it wouldn't help much for waving your arms around, it would allow you to have a legitimate conversation as your avatar. Imagine how cool it would be to do dedicated massively multiplayer gaming where you could read other people's expressions and faces, in addition to just hearing their voices.

MS is also in a unique position to integrate the touchsreen into a phone, as it controls the OS, the gaming ecosystem, and the hardware already. Sony really only controls the hardware of their phones, and their playstation OS doesn't have a phone app ecosystem to take advantage of.

Because of these reasons -- MS has a unique opportunity to do what no other company currently has the resources to pull off. Sony is saddled with Android and is also floundering in terms of direction. Apple is interested in touchscreens that also play games, and don't seem to be interested in going after gamers per se. Nintendo doesn't have the chops to make a phone.

Touch-buttons on the bottom of the screen are often accidentally pressed. So I prefer real buttons over them. But because real buttons are more expensive than touchscreen buttons and break more often, hardware manufactures prefer touch buttons.

But slide gestures would be a compromise between hardware manufactures and customers. You don't press them accidentally and you don't need no extra buttons.

You don't have to have a slide gesture areas all around the screen like the N900. Just at the bottom of the screen where the buttons normally are would be sufficient.

Another great profit of this would be that the phones without buttons look much nicer.
Here is a example:
http://img441.imageshack.us/img441/6607/unbenanntbgj.jpg

Easy and intuitive touch gestures would be
slide right: forward
slide left: backwards
slide up: home screen
slide down: opened applications
pinch: search / voice search