CMMG has developed a .22 LR conversion kit for the PN P90 and PS90. It takes just 30 seconds to swap in this kit (no barrel swap is needed).
The current design uses Ruger 10/22 magazines (somewhat defeating the point of the P90) but a high-cap model is planned.
No release date has been announced.
[ Many thanks to Ray for emailing me the link. ]Related