MBP 15 Inch - battery question?

I have had my MBP for about 2 1/2 yrs - I have Apple Care. I use it about two to two and a half hours every day with most use (65 - 70%) while plugged in and the remaining use is on battery power. About every 10 to 14 days I make sure I run the battery down to the point the window opens telling me to save everything that I am low on battery power - then I plug it back in to the charger. This pattern of usage has been good so far and the battery status icon will routinely show a 100% charge when I plug it in to charge it. Now the issue:
within the past month the battery will charge to 98 to 100% but depletes noticeably faster when operating on the battery, from 98 to 100% charge to about 60% charge. Then it depletes really fast from 60% charge to right around 20% charge and then with no warning it shuts the computer down - very fast. Everything is fine one second and the next second the screen is black and the computer is off. When this happens I can plug it in to the charger and boot up just fine.

Here's the 15 inch MBP model I have: Mac OS X, Processor 2.4 GHZ Intel Core 2 Duo, Memory 2 GB 667 MHz DDR2 SDRAM.
When I use the computer (plugged in to the charger or on battery power) all I am doing is surfing the web, e-mail, or working on Excel files. I am never running anything off the USB ports or doing anything that should use a lot of power. And finally - 99% of the use of my MBP is on an iRain aluminum cooling stand and in an air conditioned (or heated/winter) environment. It never seems to run very hot.
Here's what the "Power" info tells me: Note by "Condition", it says "Check Battery" - what does that mean - I mean how do you check your battery? Is the MBP battery covered under Apple Care or is all this normal and should I expect after 2+ years to purchase a new battery?
Thanks,
Bill

Battery Information:

Model Information:
Manufacturer: Sony
Device name: ASMB012
Pack Lot Code: 0000
PCB Lot Code: 0000
Firmware Version: 0110
Hardware Revision: 0500
Cell Revision: 0303
Charge Information:
Charge remaining (mAh): 1930
Fully charged: No
Charging: Yes
Full charge capacity (mAh): 1930
Health Information:
Cycle count: 191
Condition: Check Battery
Battery Installed: Yes
Amperage (mA): 6
Voltage (mV): 12624

System Power Settings:

AC Power:
System Sleep Timer (Minutes): 15
Disk Sleep Timer (Minutes): 10
Display Sleep Timer (Minutes): 10
Automatic Restart On Power Loss: No
Wake On AC Change: No
Wake On Clamshell Open: Yes
Wake On LAN: No
Current Power Source: Yes
Display Sleep Uses Dim: Yes
Battery Power:
System Sleep Timer (Minutes): 10
Disk Sleep Timer (Minutes): 10
Display Sleep Timer (Minutes): 5
Wake On AC Change: No
Wake On Clamshell Open: Yes
Display Sleep Uses Dim: Yes
Reduce Brightness: Yes

Hardware Configuration:

UPS Installed: No

AC Charger Information:

Connected: Yes
ID: 0x0100
Wattage (W): 85
Revision: 0x0000
Family: 0x00ba
Serial Number: 0x004ce678
Charging: Yes

Even though you've mentioned nothing of calibrating your battery ever, I don't believe that would be a factor, but 2.5 yrs is still a good run.
The battery isn't covered by Applecare unless its faulty.
You could take it in to have it tested, along with your Mac.
No harm could come of that, and it could be another problem.
I would start by calibrating your battery and resetting your SMC.
After having it checked out, then you will have a definitive answer, and know whether or not you need a new battery.

6string,
Thanks for the reply! I have calibrated the battery but it's been a while and I am not very knowledgable in computer/technical issues. How do I calibrate the battery or where do I find instructions on doing that?
Thanks
Bill

6string,
Thanks again! I have been "Calibrating" the battery but I was not sure that what I was doing was called "Calibrating" - if that made sense? I calibrate once or twice a month.
I guess it's just time to buy a new battery but before I do I will take it to the Apple Store and let them check the battery. Beautiful place you live - visited there once many, many years ago on an R&R from the jungle of Viet Nam.

Possibly calibrating a little too often there, but I wouldn't think that's the prob.
Definitely worth the trip to Apple first.
Is a beautiful place here... and you are one of the handful of people I've come across, who can say "back in Nam"