It sounds like your battery is worn out. You can find out the cycle count using these instructions
http://docs.info.apple.com/article.html?artnum=305405
Mine started to crap out somewhere between 250 and 300 cycles. When it happens, it goes downhill very quickly - I was getting about 2 hours and then suddenly it fell to less than 45 mins and got progressively worse over a few days.
I am on my third battery since 2003, although the second was recalled because of exploding risk. It's annoying that they aren't even covered under Applecare, unless they fail prematurely.