What I've Learned: MacBook Batteries Cure All Ills
For Non-Denominational Gift-Giving Holiday my mother gave me a new battery for my 3+-year-old MacBook. The old battery, which had gone through too many hundreds of loadcycles, would arbitrarily die once the battery drifted below 54 minutes of charge.
Sometimes it would last until there were 9 minutes left and then go nicely to sleep, other times it would completely power down with 54 minutes of charge remaining, merrily destroying work. Additionally, the MacBook was having trouble coming out of sleep nicely (i.e. it would get stuck in sleep mode and need to be forcibly powered down and then restarted, or it would come out of sleep but the LCD would not get any power [if you held the laptop up to bright sunlight, you could see the screen, mouse around, etc.] ), and occasionally would get stuck while powering up, or take unusually long to do so.
A little Googling makes it look like sleep/power-up problems like these can foretell all sorts of Bad Things, generally cured with expensive and time consuming logic-board and power-management system replacements. In my case, replacing the battery fixed everything. So, FYI; sometimes battery problems are just battery problems.
(coconutBattery is a handy app for diagnosing just what's up with your battery.)