I just made the leap to Moodle 1.9 this week (Mar 8, 2008) and at first seemed to be having a problem with Nwiki. When I looked in my wikis they appeared empty. I noticed a "change student" menu as if in student group mode, but it was set to No Groups. I've never used wiki groups in my main wikis.
I figured out that I had to change the Student Mode to Students in group for wiki content be visible and left Group Mode set to No Groups. I guess my logic was backwards because I thought this setting then made student content visible only to students in their same group (though in my situation I put in 95% of the content).
Comments? Maybe this will help some other confused soul.
Of course I made nwiki backups before migration!