Media Temple, a well known hosting provider, has recently rolled out a beta version of their control panel made just for the iPhone. Judging from the number of folks who sent this to us (thanks to all of you!) there seems to be a large, and until now, untapped market of people who want to be able to register a domain from anywhere.
Aside from registering domains you can also restart services, pay your bill, add email addresses, and submit a tech support ticket. I'm not a Media Temple customer myself, nor do I own an iPhone (and we all know how I feel about iPhone specific web apps) but I must admit this looks slick. You can access the iPhone version of Media Temple's control panel here.