Maybe that's a difference between Powerbooks and iBooks. My Powerbook usually runs 114 F, or 46 C...a couple degrees hotter if I use an external monitor, and about 52-55 C if I'm watching a movie or playing a game.
The fan kicks on at 47 C, and kicks off at 45 C. Buy an iLap (it's 50 bucks, so it's not exactly cheap, but definitely worth it if you value comfort). If that's too expensive, then I would suggest buying a sheet of metal that covers the base of your computer (it helps dissipate the heat faster; but only buy a good conducter).
Though, I gotta be honest, 55 C all the time seems a bit high. I would also try getting ministat2 (a widget) to monitor your battery temperature more closely. Before I got the iLap, my battery was always hovering at 95-96 F, which is too hot (it kills the long term usage). Now it's at 89-90, which isn't a huge difference, but puts the temperature in a range that's acceptable for Lithium Ion batteries.