It depends on how you use your phone. It might seem like you're losing your charge faster when you've got the phone configured to show the percent of charge remaining. I went from 100% charge to 95% charge while playing Flight Control on my 32Gig 3GS while on the crapper last week, and I thought that was a lot, but in reality, it's really not too much. Once my phone got used to charging and discharging, things seem a lot more stable as far as my battery life goes. Even using it normally, checking e-mails, shooting some videos, taking picture, iPod, internet a few times a day, I can still come home at the end of a work day with 50% or more battery charge. If I use it a little less, I can almost go two full days without recharging.
I'd say to take some more time to get used to the charge cycles. Might try to fully discharge it and recharge it a few times. Then the battery life should stabilize a bit more. I doubt you have anything to worry about, though there have been some complaints about crap-tastic battery life on the 3GS. Not sure if those people got phones with bad batteries or not though. To me, the battery light seems comparable, if not slightly better than my old 3G iPhone. As long as I can get a full day of normal use, which usually entails calling and a lot of e-mails, it's good enough for me, and my 3GS is doing more than great for that. Tack on some photos, video and internet usage, and I've found nothing to complain about yet.