I haven't noticed any improved battery life in 10.6.3. However, I have noticed much cooler operating temps and better video performance. Basically, OpenGL is running twice as well as it did before 10.6.3 on Snow Leopard. I am still getting about four hours with display turned down and Bluetooth off. Someone else mentioned possibly better battery life between charges, but I specifically checked it twice and noticed about the same battery as before 10.6.3. I didn't notice any difference between Leopard and Snow Leopard before 10.6.3 other than OpenGL being terrible in Snow Leopard. Snow Leopard premiered a 30%+ drop in OpenGL from Leopard to Snow Leopard 10.6, 10.6.1, and 10.6.2.
If anything, 10.6.3 is a huge step in the right direction, but I don't know that it means better battery... in any way.