I think it is just supply and demand.

If I am willing to pay $15 for a bitcoin then to me its value is $15...

What determines the price of honey?  What people will pay for it...

go here and click on the bottom right (HELP)
http://mtgoxlive.com

This is a cumulative graph of the outstanding orders on the MtGox exchange. It shows how much BTC/USD will have to be bought/sold before the market rate hits a certain USD price. For example, let's say you want to know how much USD would have to be spent buying BTC before the price reaches 40 USD. First find 40.0 on the bottom axis. Then find where the blue line intersects $40.0. Now find what value that corresponds to on the right axis. If it corresponds to $200000 then that's how much USD would have to be spent buying BTC before the price reached 40 USD.

Thanks for the replies guys.

Yes, right now, speculation determines the value...almost entirely. Eventually, if bitcoin continues to gain real traction as a transaction mechanism, the value should be determined by the portion of GDP being transacted with BTC compared to the supply of BTC.

Like the others, I agree that demand is what determins the price of BTC.

When I look at it a bit closer, I think demand is currently based largely on two factors: The utility of BTC and speculation. Utility is the current ability to use bitcoins to buy things that you want and speculation is the belief that BTC may one day be much more than they are now, i.e. future utility.

There are always fundemental drivers, intrinsic values so to speak, to anything that is currently - or is trying currently - to become 'currency'. In the case of the USD it is the 'law' i.e. the force of the US Government. It is people's familiarity with it and status quo. In the case of BTC it is people who are trying to set a new medium for exchange.

So as others put it, the BTC community who is going to decide whether or not the BTC succeeds.

I agree with the other members that its supply and demand. Also, greed and fear really dictate this market to the extreme. Once we see more liquidity in Bitcoin the value will have a better sense of stability.