It hasn't hurt the battery that I can tell, but I can say my wife's MacBook Pro runs for well over 6 hours, even with the 9600GPU active and WiFi enabled, under OS 10.6. With Win 7 running in a VM, it runs about 5 hours (much heavier RAM and disk IO lolad running 2 OS concurrently, so that's not surprising). However, that VM is actually a boot camp partition, and if I boot into it directly, the machine won't last 3 hours on a full charge. It's not recommending battery replacement, but it runs it down way faster... I can confirm, it;s not just a bad sensor, as once Windows 7 goes down, booting into OS X confirms the battery only has a few minutes left in it.
Win 7 is recommending battery replacement on another older notebook i have, but I can't say that's a problem as it won't run 5 minutes anyway on a charge, that battery's long since passed it's usable life... i use it exclusively while plugged in. i only keep the battery in it as it gives me a cushion on usabiltiy if I have to change seats in the living room (and thus outlets) or if our 2yrold gets a hold of it...