I would like to give advice for traveller's going to Kenya since I hear still many people are booking a safari or getting prices in advance.
It really does make a difference. For example a 3 day safari all-inclusive including very good lodges to Tsavo can be booked for around 420 US Dollar per person. Booking in advance can cost you around 600 US Dollar per person. If you are a couple you can safe 360 US Dollar in total!
We were staying in December 2010 at Southern Palms Beach resort and were able to book a 2-day safari at Voi Safari Lodge for 180 Euro per person, but decided to book a 3-day safari instead to Tsavo and Amboseli for 320 Euro (420 US Dollar) per person. We stayed at top end lodges (Sentrim Ambosili and Voi Safari) and had a 4-wheel drive bus. You will have to bargain ofcourse on the price, but it is worth it. Across the street from the resort are small shops run by the locals. If you are staying in this area you can ask for Big Joseph who will arrange everything for your safari. Otherwise have a look on this website located near Barclays shopping centre in Diani Beach.