Paypal Express Checkout API, Google Checkout API and Amazon Flexible Payment Services allow us to accept payment easily by simply redirecting users to their website and they'll handle everything. We don't even need to know their credit card numbers.
However, what if I want my users to deal with me directly using their credit card numbers? This way I can change the payment processor without bothering my users. I think this is how traditional merchant accounts work. Does any of the payment services offered by Paypal/Google/Amazon work in this way?