Hi, I got almost the same question as you did. I post it on this URL:http://www.dotnetnuke.com/Community/Forums/tabid/795/forumid/111/threadid/161142/scope/posts/Default.aspx
and I'm still waiting for answer.
I suggest you change the Button to a LinkButton and check it if works on Page Edit Mode, If it works on Page Edit Mode but NOT WORK on Page View Mode, you and I have the same question.
I think it's DNN's total security mode cause this.