Nederfox:
"Wrong. Many NICs and routers have a highly user-configurable MAC address. So this does not have to be unique"
I'm afraid you're wrong. Have you studied Networking? Do you know who the ISO is? ALL MAC addresses ARE unique and I would gladly accept proof otherwise! It's a networking standard and MUST be true in order for networking to even be possible. Granted there may be an incredibly small chance that 2 people will have the same MAC address but the odds of getting struck by lightening 3 times in a row are greater than finding 2 people with the same MAC address. It's a Law, Standard, and Must Have function for networking. Any test you take will tell you this. MCSA, MCSE, MCP, Network+, Security+, etc. MAC addresses can be altered through routers and programs but when packets are sent, the burned MAC address is always the same to each NIC, regardless of alteration. Change your MAC address with whatever program you use and then do an ipconfig /all and tell me it's not the same.
A MAC address is just like a social security number or VIN for your car. You need to do your research a little better on this issue. The address you are thinking of is a MAC clone address... not the MAC address. There is a BIG difference.
As far as chained proxies... yes it will make tracing more difficult but that's why we have computers. So we can let them sit and do all the work for us.
Even through chained proxies, one can be traced in a matter of minutes if not less. I'm just basicly saying that it's not worth it.