Well, I do get very close to 8 hours on my 17" MBP (when the cpu is not too stressed). That number will go down if any serious work is done, but light browsing etc. takes it all the way up to 8h.
The key I find with my mid 2010 17 is to manually force intel graphics. Several web browsers and plugins seem to invoke the NVidia discrete chip which sucks power.
