lately,the bitcoin price has been go down ,why? what happen important event

I think the $8-10 range is probably the true value of bitcoin at the moment. The only reason it jumped to $11.50 is because of the speculation that the BTC conference was going to somehow drive up the demand for BTC, but it kinda didn't do anything so the price dropped back down again.

At least it looks like it's creeping up a little bit after a week of being down below the 2 day average:

http://bitcoincharts.com/charts/chart.png?width=1107&m=mtgoxUSD&k=&r=2&i=Hourly&c=1&s=2011-08-26&e=2011-09-02&Prev=&Next=&v=0&cv=0&ps=0&l=0&p=0&t=S&b=&a1=SMA&m1=48&a2=&m2=25&x=1&i1=&i2=&i3=&i4=&SubmitButton=Draw&