I've had my MBP now for just about 3.5 years. It's a 2.4 GHz C2D, 4GB RAM mid-2009 model. It's currently running 10.8.2 (I keep up-to-date quite religiously).
Anyhow, my battery life has always been quite decent -- lasting variably between just under 2 hours and 3.5 hours. I try to be quite cognizant of what is "good" for the battery by avoiding constant charging and occasionally letting it drain to 0% in order to recalibrate. In the last several weeks, it's "condition" appeared to start deteriorating..at least according to Mac OS X's discretion, and changed to "Replace Soon." This worried me, so I installed the "Battery Health" app and consulted iStat Pro to see what they were telling me; both indicated that my battery was still an upper 80% in health.
This brings me to yesterday. I had taken my MBP all over with me throughout the day with no issue, and then brought it back to my apartment that evening. I left it charging and took a 1 hour nap. When I woke up, the charger indicated green but the battery indicator was flashing when I clicked it; when I opened my computer, it had a "charged" symbol but was at 0%. The condition had also suddenly changed to "Replace Now." In other words, if I disconnect the charger at all, the computer immediately dies.
Do y'all have any advice about what could have caused this so suddenly and dramatically, and whether it's at all remediable without getting a battery replacement (this model doesn't let you easily replace the battery)? To speak the obvious, it's incredibly inconvenient and ultimately untenable, but I'm not sure if I can hold Apple at fault since it's obviously OOW.