Sandman2 sends word that in the April 1998 issue of Gamepro reports that MK5 is already in production and will be based on the successor of the Zeus graphic chipset, the Zeus II. TRMK reported back on December 21, 1997 that MK5 would start production in 2 months. This article in GamePro put an end to those who doubted it's veracity. Here's the article straight from Gamepro.

Midway has confirmed that two of its hottest arcade franchises are already ramping up with sequels. NFL Blitz ‘98 improves upon the raucous original by increasing the number of human participants to four. Meanwhile, it should prove as no suprise to anyone that Mortal Kombat 5 is officially in the works, this time based on Midway’s ZEUS II hardware, the still evolving successor to MK4’s circuitry.