When I bought my new set the salesman talked to me about industry standards re: sagging. I only half listened but got the general understanding that it is acceptable (to the industry) for mattresses to sag up to an inch or more. This 2008 article speaks to that:
************************************************** ***
Here are a couple of other articles that might be of interest (the 2nd link to "Sleep Like the Dead" has GREAT info for anyone looking for a new mattress):
How to Measure Mattress Sag
Mattress Warranties: What You Should Know