Cheers stakhanov!
Currently, it's simply using the last price on Mt Gox. I'm planning to alter the calculation to include other (higher volume) exchanges.
The number of exchanges isn't that important. Mt Gox has 90% of the volume.
A better algorithm would be one which is volume weighted over a period of time.
Say in last hour there were 5 transactions.
1000 BTC @ $3.99
200 BTC @ $4.06
800 BTC @ $4.01
1 BTC @ $4.80
obviously the $4.80 is an abberation but it is what users would see.
Taking a weighted average (volume * price) / (total volume) = $4.01 (technically $4.005397).
The only thing to consider is how large of a weighting period. The shorter the period the more reactive the average. The longer the period the more it smooths out the spikes but the longer it takes to react to sustained price action.
For a more "immediate value" I think 1 hour would be good starting timeframe but you may need to adjust.