I've played around a bit using the smcFanControl which really just allows you to increase the minimum default. It allows you to have 3 different settings based on using it on battery, AC (not charging), and AC charging.
I currently have it set to:
battery: 1800 rpm
AC (not charging): 3100 rpm
AC (charging): 4600 rpm
These settings keep it at about 45 C for light IM/e-mail/web activity level, except on battery it runs about 50 C.
My reasons for these settings are when on battery, a slower fan decreases power usage. 3100 rpm on AC not charging is just low enough that I can't really hear the fans, but keeps the cpu the coolest. Increasing it for when the battery is charging allows the battery to stay cooler since charging will heat it up more.