I was going to do this in PHP using the mtgox API but I actually think it's a much simpler project.
Basically I want to put in my website the price in GBP. I'll wrap these prices in a <span> with a class of "price". Now I'll put a button on the site to convert the prices to Bitcoins. Then it'll just fetch the exchange rate from mtgox and do a small calculation to change them. I'll post the code on here when I'm done.
Anyone got any ideas about how this might be done? Or if it's doomed to failure?