You now must include a numeric value for the page parameter.  In the past, if you omitted a value for the page parameter, AWS's default behavior was to return the first page of results.  AWS now requires that you explicitly set the page parameter to the number of the page you wish to retrieve. 

For example, if you made this request in the past:

http://xml.amazon.com/onca/xml3?t=[YOUR_ASSOC_TAG]&dev-t=[YOUR_DEV_TOKEN]&KeywordSearch=star&mode=dvd&sort=+pmrank&offer=&type=heavy&page=&f=xml

You should now make this request: 

http://xml.amazon.com/onca/xml3?t=[YOUR_ASSOC_TAG]&dev-t=[YOUR_DEV_TOKEN]&KeywordSearch=star&mode=dvd&sort=+pmrank&offer=&type=heavy&page=1&f=xml

Laura at Amazon