What surface are you using the iBook on? If it's your lap, or other non-hard surface, and the temperature in the room (area) is fairly high, the heat level is normal.
If, however, neither of the above apply, it's your call if it's too hot. Over the Internet, it's hard to judge what's too hot and what isn't. If you're really, overly concerned, get a technician to look at it.
Mac mini (Mid 2011) 2.7 GHz i7, 8GB RAM, Crucial M4 256GB SSD + 500GB + 1TB FW800 OWC Mercury Elite Pro mini
iPhone 4S • iPod nano 8GB • Sound System Audio Engine A2 • Display UltraSharp U2412M 24"