We
kind of noticed this earlier, but not in any coherent capacity. The
affiliates page on LJ was
added on 2006-01-13. The main
Six Apart affiliate program page lists TypePad, Moveable Type, and LiveJournal.
Here's their post introducing it. If nothing's changed since the last time we checked those links, the affiliate links lead to
this start page, which still has a wee bit of outdated information ("ScrapBook photo hosting with 1 Gig of storage space").