I have a MacBook Pro (about 18mts old) which has recently started to inexplicably die without warning. I'm running it on battery and, yes, it's getting close to the bottom end (between 10 and 25%). In the past I would get to about 10 minutes of life left and the "Reserve Battery" warning dialog would pop up and let me know that I'd better get on the adapter. 

Not any more.

Now it gets to about 25% and then plik! it's black screen and dead. Instantly. No warning, nothing. 

**Some details about the Mac**

* Hardware: 2.5Ghz Intel Core 2 Duo 4Gb, 667Mhz DDR2 SDRAM
* Software: 10.5.8
* Model: 4.1
* Boot ROM: MBP4.1.00C1.B03

Following advice from @Alex, I've checked the system logs (Console) and disk verification tool (Disk Utility) but they both come up clean. Apart from the instant death at low battery, there are no other dubious intermittent faults.

**Edit to cover suggestions so far**

Many thanks to all who have taken the time to address this issue. I've summarised the answers so far:

* Look at the battery cycle count (About this Mac | More Info | Hardware - Power - Battery Information) - @glenstorey
* Recalibrate the battery - @Lawrence Velázquez
* Check hardware (RAM seating, hard disk etc) - @Alex

**Final result**

The whole thing about battery cycles being guaranteed by Apple appears to be a myth (at least the bods at my Apple Store were unaware of it). So I changed the battery and all was fixed. Back to normal.