I've followed these instructions which should allow old BackBlog comments to remain visible (if, indeed, their service returns as all I'm getting is "Service Unavailable" at present). Although I don't think I'll have to, I may need to manually re-publish some of the older posts in order to see the old comments. If so, my RSS feed may be clogged with past posts. If this happens, my sincerest apologies!
Update (Mon, 29 August 2005, 10.25am): I had to manually repost about 200 entries to keep the old comments (*sigh*) but they're all done. Hopefully RSS spam was kept to a minimum ... if you saw 200 posts, you have my sympathies!