jnairn -
First, I edited your post. We don't want that number in a public forum :).
Now, the reason you are seeing that is because of the type of field you chose. An integer is too small to hold that value as a number (see http://en.wikipedia.org/wiki/Integer_overflow if you are interested).
You'd be better off just making the number a text field. So just delete the field and recreate it. Give it a size of 255 characters just to make sure it can handle my next point...
Finally, best practice is to encrypt any sensitive data that you are going to store raw credit card numbers in Method. If you are going to be storing sensitive data, you should loop though your data and update it using the encrypt action to make it unreadable, and then right before using it run the decrypt action to make it readable again.
Paul