According to Ruliweb.com Paper Mario DS is set for a July Japanese release, which isn't too farfetched, quite conservative actually...

And the game might be announced properly at E3 this year.

There's been no official word from Nintendo as of yet, but the retailers may know something we just don't.

This is all good news anyway, because I throughly enjoyed Paper Mario: The Thousand Year Door on the GC, and I have the Wii game, I just haven't actually played it yet... You can blame the excellent No More Heroes for that.

Let's hope that we get Paper Mario DS soon, if it really does exist.

Source.