I believe that your understanding of PCI compliance is correct. The
organization I work for is working hard to become PCI compliant, and the
requirements are quite strict (and annoying). And it's probably more
than a questionnaire... your answers to the questionnaire must be the
right answers, and you'll probably need to pay an outside company to do
penetration scans and things like that. I'm not aware of any free
scans. Periodic code reviews are also part of the equation if you
process enough transactions to cross certain thresholds.
-Nathan