Depends. How old is the battery? What is the use pattern?
If the battery is a couple of years old, and particularly if the use pattern is to run the battery all the way down each time before charging, then the battery very well may be worn out.
A battery can fail or show symptoms of failing in different ways. One is to take a charge and show fully charged, but then run down quickly.
However, there are other things to look at. Running apps, particularly apps that hit the network for updates all the time are notorious for short battery life. Likewise some navigation apps, when left on. Also, being in a low, in and out, mobile signal location can run a battery down fast. Finally, some models are worse than others with wifi and bluetooth.
My advice is make sure all apps are shut down. Pay attention to any app that was downloaded before the short battery life was noticed, and don't forget apps that are set to run in the background when closed. Fully exit everything. And turn off wifi and bluetood, and pay attention to mobile signal level. Then test for a day.
If after ruling everything else, try a new battery. Shop around. You don't need to pay full retail, but buy a genuine BlackBerry battery from a reputable source.
T-Mobile will replace batteries up to 90 days from purchase however even though all other manufacturers have a one year warranty on batteries RIM/Blackberry in my own experience does not support that, however all is not lost, as aiharkness said make sure you buy an official blackberry battery (blackberrys are very picky) and get it from a reputable source my best advice would be amazon.com they have blackberry batteries and they range from 3-8 dollars.
Hope that helps,