This could be fuzzy math, but here's one attempt at giving you an average power usage number.
Doing normal things (surfing the web with iChat and iTunes), I get about 3 hours of battery life. Apple claims the battery is a 55 watt-hour battery. According to Wikipedia, one watt-hour is equivalent to 3,600 joules. A watt is defined, according to Wikipedia, as 1 joule/second.
Thus, 55 watt-hours is 198000 joules, 3 hours is 10800 seconds, and the power consumption averages 198000/10800 watts = 18.33 watts.
I could be completely wrong on this, but I think it's about as good an answer as you'll get, unless someone has a Kill-a-watt power meter.
I have a 2.0ghz Macbook by the way