I am writing a little order form for my site, I have been getting a fairly large amount of suspicious orders and I'm trying to get ways of minimizing the risk of fraud.

I would be attempting to run some stats against your orders. If you build a profile of slightly, but not quite the same orders (for example all the credit cards coming from the same bank), you could choose to verify each of the suspect orders, either using the bank, contacting the customer directly, using email etc etc etc

If you are already suspicious, then something is slightly wong (or at least interesting). Think hard about your suspicions, write them down, design a solution then automate. Obviously carbon units are very good at fuzzy logic, so creating broad rules that raise exceptions you (the carbon unit) have to filter may be the best method..