DotRegistrar is the only one I've found that has this feature. I've used it plenty-o-times and it's paid for itself - well, not litterally... SSince you have "domain credits" in your account at all times it's nice to be able to register a domain on the fly without having to enter creditcard info every time.
I still have domains I haven't used but at least I had 3 days to think about 'em
