Posting thought returns the laconic "Sorry... We're currently experiencing some technical difficulties. Please try again in a few moments.". Doesn't seem like a known issue though on lj_maintenance.
When this doesn't happen, I get a 400 Bad Request with: invlid characters following the protocol string.
UPDATE: Fixed the problem in my LJ sync by removing Keep-Alive. There seems to be nothing wrong in the request, but looks like something wrong on the LJ side consuming it.
UPDATE: Another change from a few weeks ago is that the posted XML must have the entry in the xmlns="http://purl.org/atom/ns#". Otherwise you get an internal server error.