20.0.58 according to All About Symbian:
It seems that after a gentle kick from yours truly Nokia N93 firmware updates via Nokia Software Update are finally back online after 3 months downtime. Moreover, the next-gen (masses more RAM, WLAN wizard, Search, Download!) v20.0.58 firmware is finally available, though watch out for a nasty camcorder bug.
I’ll definitely upgrade to this once I’m done installing Windows tonight.