Rotation = no. Sliders are just as bad, they're not very durable designs and it's far too easy to break the two halves apart if it's dropped or caught at the wrong angle while open.
I've never understood sliding designs really, even in mobile phones. Why choose to protect the buttons but leave the main screen exposed? I'd rather have a clamshell.
How about the Game Boy Micro?My favorite has to be the shogmaster one. Only thing that I'm disappointed with is that there isn't a GBA slot. Every previous nintendo hand held has had backward compatibility, it's hard for me to think that I woun't be able to play my GBA (and maybe even GBC/GB) on my new DS.
Maybe I'm thinking too much into concepts... but I'd still buy these if they were made.