I am using RC6.
I have the shipping address turned off, because I only need the billing address. Also using the contrib where it removes the order summary page, if that's relevant.
If you have the billing information filled out, but there is an error with filling in some part of the credit card, after the form rebuilds, the billing information is cleared out, and doesn't have the previous values. This has the potential to be very annoying and kill conversions.
However, if one of the form errors involves the billing address (say you don't put a zip code), then the fields are populated fine when the form rebuilds after the error.
Very strange. So as long as there is an error on the Billing information as well, it works fine.