I've been doing a lot of research on this and think I have a much clearer picture now.
There are a couple of details that have emerged.
As Chris says, if your website never sees the cardholder information (i.e. the transaction is completed on the PayPal website) you are basically in the clear.
When the entire card handling business is outsourced in this way you are SAQ-A under the PCI rules.
If you accept and process credit card details on website (like PayPal Pro) then you are SAQ-C.
A point to note about this is that
if you are using a Shared Hosting account you are probably ALREADY breaching PCI. According my own hosting company "SAQ-C compliance can never been achieved on a Shared Host". You should be operating on a dedicated server (expensive, I know!).
If you are storing Credit Card data in electronic format (can't imagine why you would want to) then you are in a world of pain already because you are SAQ-D which has a 200 page list of compliance criteria such as keeping 3 months worth of CCTV surveillance footage and a bunch of other crap that really only Enterprise scale companies have the resources to deal with.
If that isn't enough to make your head spin;
from 1st July 2010 any merchant using a shopping cart system that hasn't been PA-DSS certified will be in breach of SAQ-C or SAQ-D rules (SAQ-A is still sweet since your shopping cart is outside the "scope" of the PCI rules if it never sees the card data).
Pretty much anybody taking card data on their shopping carts whether it be open source, home brewed or non-certified commercial needs to think about an exit strategy right now.
At present the only major (affordable) PA-DSS certified shopping cart is Pinnacle.
X-Cart is working towards it but probably won't make the 1st July deadline.
Magento Enterprise is already there but at 9,000 bucks per year it's not feasible for small / medium business.
PayPal Standard / Google Checkout / Amazon Payments are looking better by the day?
Check out page 28 of this PDF doc for a diagramatical summary
https://www.pcisecuritystandards.org...uick_guide.pdf
and this 7 page White Paper by PayPal themselves basically encouraging developers to encourage CLIENTS
to move across to either PayPal Standard or PayPal Express.
http://cms.paypal.com/cms_content/CA...WhitePaper.pdf