We're observing short-sighted strategy as well as tradeoff between customers and developers. Sony wants to convince people who want /desirable feature/ or /desirable price/ that the product with that feature or price will sell out and be gone, implying the other product will be the only one available and they will have missed out on feature or price. Examples: the 20GB model, the 60GB model, and backward compatibility.
This is short-sighted because it will not affect sales in the long term and will alienate customers a greater extent than simply being honest (although that would not boost sales). Tricking people to boost sales will make many of those people feel upset unless they're very satisified with the product.
Sony needs sales desperately /right now/. They need to keep third parties interested, and they need to keep the games flowing. They're falling behind with third party support compared to the 360 and especially the Wii. 3rd party support is what made the PSX and PS2 awesome systems and a lot of Sony's most important partners have been rumbling about Sony's poor sales -- Square-Enix and Konami in particular.
This is the customer-developer tradeoff -- they might imply things that aren't true to get customers to buy the system now if it means they'll maintain or improve developer support. The customers get a little more upset since the unit they're buying and being told to rush out and get becomes obsolete at the same price or drops in price a month or two later and the developers get a little happier since the installed base grows at a more promising rate.
This sleight of hand cannot be maintained forever.