I would use the weighted average of the last trades at Mt. Gox. The trades can be downloaded very easily in JSON format.
I'm pretty sure the logo is free to use, but I'll leave it to others to give you a definite confirmation.
Yeah, it's tough to set out prices at the moment with the volatility... I'm no expert in economics but I think the value will stabilize if there's more coins in circulation, so it's just a matter of taking some risks at the moment so you're in a position of power when things stabilize.
We were using the value of "last" at mtgox, but we've switched to the 24h weighted average via bitcoincharts, which you can get programatically here: http://bitcoincharts.com/t/weighted_prices.json
It's still possible someone could take advantage of you slightly on the backside of a big downswing after a long flat period, but the risk is acceptable to us because of the tiny amount of interest we've had so far in people buying Mumble servers for Bitcoin (ie, if someone sneaks in a trade to us while it's at ~$25 and the price comes back down to $10, we can afford to "lose" that - well, really all we're doing is selling a service to one person at a discounted rate for a while). I haven't bothered to hack together an automated process yet, simply because my plate is already full and interest is so low (I'm not exactly getting swept off my feet accepting BTC trades) but I'm confident we'll get there eventually.