MacBook Pro Cycle Count means the number of times a battery’s entire power is used up. It’s formally defined as:
A charge cycle means using all of the battery’s power, but that doesn’t necessarily mean a single charge. For instance, you could use your notebook for an hour or more one day, using half its power, and then recharge it fully. If you did the same thing the next day, it would count as one charge cycle, not two, so you may take several days to complete a cycle.
It can be easily determined by clicking on "Apple", "About This Mac", "More Info…", "Hardware", "Power", "Health Information:". MacBook Pro with a replaceable battery retains 80% of its original capacity after 300 cycles as mentioned here. But in all practical cases, I’ve heard users replacing the batteries closer to 300 counts. This number goes upto 750 for MacBook Air and 1000 for newer MacBook Pro so there is relief already.
My MBP cycle count hit 283 on Friday and the scary part was "Full charge capacity" was down to 258 mAh. In usage terms, a fully charged battery was getting drained out in 10 minutes 
A new battery was rushed, installed and the new count is certainly the expected number:
Here are some more relevant docs:
- Calibrating your computer’s battery for best performance
- Replacing the battery in your MacBook Pro
- 8 Ways to Help Your MacBook Battery Last as Long as It Can
Technorati: osxtips battery cyclecount
From my own experience and what I heard from others, Mac batteries are extremely bad. I own(ed) many laptops, and only had to replace batteries of those from Apple within their first two years. Never happened with other brands, even those cheapo ones. Usually they can keep 80% or more of designed capacity even after a couple of years of daily use. Not with Apple. Clever way to have an additional good revenue stream?
Comment by Trung Duc Tran — October 20, 2009 @ 1:17 am