Is it possible in the search feature in Dreamweaver CS3 to find all pages that do NOT contain a specific string?
I don't think so, at least not directly. I could be wrong, Regular Expressions can be pretty powerful, I'm not sure how well they're implemented in CS3 though: http://www.adobe.com/devnet/dreamweaver/articles/regular_expressions_p t1.edu.html
Maybe you could do a find/replace of that specific string, with itself.
All of the files that have it would be updated without actually changing the code.
The Date Modified for the file would change though.
The files without the string of info you are looking for would have the old modification date.