I really think that this can depend on a few elements. One is the type of product. If you are selling a low cost impulse item, soliciting small donations, etc. you might benefit from a streamlined one page checkout. However, many businesses seem to get better conversions through multi-page systems. Personally, I think there are two reasons. First, on each page you get another opportunity to reinforce your security and privacy messages - by the time the user is entering their credit card information they have seen the security notices repeatedly. Another is that you get an investment. Each page feels like another step already taken, so when the customer gets to the credit card screen, well, they have already come this far so they might as well place the order. With a single page, it may feel like they are still on the first step, they haven't made that much of an investment in the purchase so they can just back out.

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The best way to learn anything, is to question everything.