After moving over from MT to 1.5g, I set up comments to be moderated. The first two new comments from visitors whose comments were in the imported comments had their comments moderated correctly. But the next commenter who hadn't commented in WP but is in the older MT comments, posted without moderation. The second commenter was new and also posted without moderation.
I'm assuming the beta is a little wonky in this arena, so for now I'm off to install a captcha solution, unless I'm missing something else to look at.