13inch MBP: Is my battery defective?

OK, so the Apple advertised battery time is 7 hours. I understand these are generally high-ball estimates, however independent reviews from cnet and other laptop mags have placed it at 6 hours if not higher while running music, the internet, video, etc.

I just did a full charge, let it go down to 0, and repeat. My results battery lasted 4 hours and 20 minutes. I had Firefox open with a few tabs, Adium, Evernote, Microsoft Word, and iTunes. I had my iTunes constantly playing at slightly over 50% volume (connected to headphones) the entire time. I browsed the internet for maybe 15 minutes and chatted on Adium for another 15. I would call this LIGHT usage. No games, no video, nothing out of the ordinary.

Is there something wrong with my battery? I expected 5 if not 6 hours!

I was at the Apple store today and overheard someone with the same problem with a 13 inch MBP. They were told 4-6 hours with general use, which includes surfing the web, wireless, low screen brightness, low keyboard brightness and no BT. I can tell you that I get only 4-6 hours on my MBP 15" models. I did have to replace the battery on one unit as I was only getting 2 hours with light use. I believe the usage you described is a bit taxing on the battery and heavier than you think. Four to five hours is probably all you will get....sorry.

I'm probably OK with it as long as it's not out of the ordinary.

What conditions were the 7 hours based on? Try replicating the conditions and then measuring time.