Have you had to use your mattress warranty? When I looked into it most warranties were pro-rated, required that you followed their mattress maintenance schedule, customer had to pay for inspection, 'normal wear' was not covered, etc, etc. This article [1] claims that actual useful lifetime of a mattress is around 1/3 to 1/2 of the warranty period.

(I ended up buying 2 awesome, cheap, non-brand name, made in the US mattresses from Restonic, which are still going strong after 5+ years).

[1] http://www.sleeplikethedead.com/mattress-warranty.html