I am relatively new to WordPress, but I have noticed this problem as well. I am using 1.0.1, and it seems that no matter what input on the edit comments screen, I can only get the last 10 comments. Has this been remedied in one of the nightlies? Is there some sort of other workaround?
I haven’t had the same problem with editing previous posts, but I haven’t posted more than 20 posts just yet. I suppose that when you are logged in and then view your site, you can click on the edit link for any post that you can see. But unfortunately, this workaround doesn’t work for the comments.
I have to say, I tinkered with movable type for quite some time, and then found a mention of WordPress in the MT support forums. I installed WP and haven’t looked back. This is a great product. I am using WP as a news blog for our library https://www.library.ohiou.edu/newsblog/ . I am also using WP for a personal/family blog as well. I am hooked. Thanks for a great product and great support.