I think its been discussed before - and we all had out "fantasy" caliber that we'd love. There was talk early this year about some sort of 22 conversion kit. But it seems that it has never materialized. And, one would think that it would require replacement of the entire receiver in order to work.
I'd love a 9mm one. But, I think spec wise, they have issues getting the gun to run with other calibers, from what I remember reading earlier this year...