lithium rechargeable batteries dont have a battery memory, like nickel cadmium rechargeable batteries, but once they have been produced they begin to deteriorate because of a secondary reaction that goes on within the battery (this applies to all rechargeable batteries). when people tell you to "cycle a battery" every so often that does not extend your battery life. from research and what i have read, fully depleting your lithium ion battery will actually shorten its lifespan, and if you plan to use your laptop without a battery (i believe that if you use your macbook w/o a battery there is a cpu performance penalty, dont quote me on this) keeping it charged to 100% and storing will wear on your batteries life span, it should be left about 80-90% if you plan to store your battery. also, another way the battery wears down is that the cells do not draw power evenly, so each cell in your battery has a different individual charge % after use, when you plug your laptop in power is directed into all the cells, even the cells fully charged, this results in battery wear. batteries are suppose to last about a year maybe a little longer depending how you use it, so dont expect it to have anything close to a 3hr charge after +2 years. after a year most people will have about 300 cycles on their battery which equivates into about 15% loss in capacity, the more cycles that you go through decreases the battery length exponentially. (e.g. my old laptop before i bought my new macbook had about 1000 cycles in two years, due to the nature of my work and uni projects, this resulted in a battery life about 20mins from my original 8hr capacity)
though the cool thing with the new aluminum macbooks, is that it has a sensor that detects the charge of each of the individuals cells, so it can charge smartly and evenly. it cuts the power to a fully charged cell, instead of bleeding power into that cell to charge another cell (which results in battery deterioration.)
i hate posting stuff like this because people will still post about how "cycling" batteries will increase battery life, like how people will argue ddr3 has a performance advantage over ddr2, at the moment there is no performance increase, just higher price (this is based on ddr3 having same voltage, but higher frequency compared to corresponding ddr2 with lower frequencies i.e. ddr3 1333 vs ddr 800, they perform about the same). the fact is lithium ion batteries in my opinion have three advantages over nickel cadmium: they have no battery memory, weight, and charge capacity. of course there are more advantages, but those are my thoughts.
*battery calibration does help your battery life, just not performing every week. just do when apple suggests to calibrate your battery