I've been think about this for a while now, about purchasing a cheap Nokia 770 Internet Tablet and running a remote control program for MCE called MCEGuru.
It looks pretty good and the Nokia 770 Internet Tablet has a touch screen!
I'm currently using ngRC on my Sony PSP which is a fantastic remote control application for Media Center but I'm not sure if further development is planned for it? I wrote about ngRC previously on my blog
You Tube Video
MCEGuru Screen shots