I registered a domain name today, my first time doing so, and experienced a brief period of panic when the billing had gone through, but no domain name was showing under my account. Also, both DreamHost and other registrars were still showing the name as available.
Of course, about ten minutes later I received an email announcing the success of the registration, the domain appeared under my account, and the name no longer showed as available for registration.
Still, I was unsure after my purchase whether I'd done everything correctly or not, or if I needed to try registering again.
I'm sure many of your customers will have previous experience with this and won't be bothered by it, but just for piece of mind for those new to domain registration, I'd suggest a text box after domain registration purchase advising of the possible wait time for the registration process to be completed, and notifying customers that an email will be sent once it has.
Other than that, domain registration was a painless, pleasant, and exciting process. Now all I have to do is purchase web hosting... after I've made the web page to host, of course.