I thought I had to write this down in case someone else out there is having the same problem. I just re-organized my aspx pages into content pages for use within a master page and started getting the infamous "Invalid postback or callback argument". So I did the usual when I recieve some strange error message, I copy/pasted it into google. What I found was thousands of similar posts from other having the same problem and no answers.

After some looking I found a post which helped my track down my problem: http://weblogs.pontonetpt.com/miguelisidoro/posts/24975.aspx. It was Miguel's comment about the nested form tags that got me going. I did a quick search of my aspx page and found that I had forgotten to remove a ending < / form > tag. So watch out for nested and or incomplete html in the pages.