People who are familiar with DNS will know that any DNS changes can take 24, 48, and I've heard 72 hours as well, to be completely visible to all internet citizens, and may take longer if they are using two tin cans and a bit of string to connect to the internet, as the holder of the string may have to manually check for any updated DNS information.
People not familiar with DNS may suspect a problem if they know they've made the changes with their DNS host and set things up at LJ, but still do not see their LJ when they visit their domain.
Any changes to your domain's DNS can take [timeframe] to be visible to everyone on the internet. Some people will see the change the instant it has been saved with your DNS service provider. It can take other people up to [timeframe] hours to see the change to your domain. This is due to DNS caching, and will be resolved as the cached DNS records expire and affected people's internet service providers check for the updated information. Neither LiveJournal nor your DNS service provider will be able to speed up the DNS propagation process. [If some poor sap had their DNS information set to be able to cache for a week, I'd imagine it would be scarily longer, too, but that's beyond the scope of this FAQ.]
(Also, the other references to "domain registrar" in the troubleshooting section should be changed to "DNS service provider", since it's not necessarily their registrar. Email service provider is another common option for someone's DNS service provider.)