From what I hear his website is divided in two parts:
1. Purchase bling like t-shirts, posters, pens you need a credit card AND the 3-4 digit code on the back to authorize.
2. To make a contribution, you only need the credit card number.
So, anyone with a few million stolen credit card numbers and access to the bank ACH system can contribute massively.
Any good computer geek can do this from anywhere on the planet.
If you require the 3-4 digit code on the CC back (as Romney’s campaign contributors are) the fraud shuts down. Guess zero is more fair than Romney.
The big question is whether or not the name and address are matched. The way around donor limits is to let me supply my credit card number and the name 'Mickey Mouse'. The FEC only gets to see the name, not the credit card number nor the safety number, so I could send in lots of money under bogus names.
Similarly, using anonymous debit cards (i.e. gift cards) avoids any name checks. This is considered a small cash donation, so no name required.
The FEC has made cheating rather easy by not moving beyond a 1950s way of thinking about money and donations.