In the example where he shows that the PayPal logo requires a bit of microcopy to explain you don't need a PayPal account to purchase, might it be cleaner to just feature the PayPal logo alongside the other credit card icons? The other example, where he specifies the billing address - well, I can't really think of a better way.

While I use it from time to time myself, my gut feeling is that if you find yourself needing microcopy, you might be missing something.

I refer back to this Nielsen Alertbox from 2005 http://www.useit.com/alertbox/forms.html from time to time. Basically, Nielsen thinks that "help" items are fine, as long as they're on the same page, in the same context, and your form is as short as possible.