If the idea is to have a price that is quoted, as the price of bitcoin, then the best option is to just do an average of averages.
For instance, if you have 10 different exchanges, you sample the price at each exchange on the hour, and average the price, having sliced the top and bottom prices from the sum.
This formula would be accurate enough to give info, but not too accurate to cause caching or logging issues. Anything more would be taken directly from the exchange being used to trade from.
If someone wanted to create a 30 day average price, they could from the figure created every hour, but for the rawest useful global number, I think this might work!
the only problem with sampling 10 exchanges is that each exchange seems to follow (like sheep) mtgox.
EG bitstamp is usually $1 more BTC-e is usually a little less.. but whenever mtgox skyrockets or plummits.. the others follow suit for no direct reasons but to stay in toe of mtgox value, thus bringing you back to a average of 10 exchanges still being the same as mtgox