http://www.livejournal.com/support/faqbrowse.bml?faqid=129 One can actually point a subdomain to one's LJ, although there's no mention of this in the faq as yet. One does this by the same mechanism that one points a domain there.
This can be seen in the wild on
http://blog.azurelunatic.net I'd propose to change the FAQ as follows:
Current:
Paid, Permanent, and Plus users can forward their domain to their LiveJournal, after a short setup process.
Proposed:
Paid, Permanent, and Plus users can forward their domain or subdomain to their LiveJournal, after a short setup process.