Writing Microcopy
bokardo.com
bokardo.com
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.
As for the Paypal thing, I hear what you're saying, but I think the microcopy should stay. It does no harm, fits just fine into the interface, and if it sways even one person who blanched when they saw the Paypal logo (next to the credit cards or otherwise), it was worth it.