I mean, is it possible to make it run a decent and playable speed?
put simply, no. its not possible... for now.. maybe If we ever hack the new PSP coming in october(I think)
Is it even possible to code?
Nothing is impossible to code

, its just a matter of if the console or system thats running it can handle It I guess.
Has anyone code one?
Im not sure if anybody has ever tried to, Probably not because it would be Months and months of work in vein.
Is it difficult?
Yes, it would most certainly be difficult, like defeating Chuck Norris at a being Chuck Norris contest. (Will never be done (on psp))
It would probably be equally as difficult as cloning or decompiling and porting the game you want to play
It would be cool though

EDIT:
Thats definately a quote worth putting in your signature

Yes, it would most certainly be difficult, like defeating Chuck Norris at a being Chuck Norris contest. (Will never be done)