What is currently the de-facto way of backing up Tumblr tumblelogs? I have "backstage" access to Backupify's Tumblr service, but (as they forewarned) it's not all too stable.
you can also try this free tool, written in java: http://code.google.com/p/tumblr-backup/
but it just backs up the photos. But an advantage is, that it does incremental backups, starts on each run where it left off the last time.
If you have a Mac, there's a Mac application available. I know that at one point there was an effort to port this (it's built on PHP scripts, so it shouldn't be too tough) to other platforms, but I guess nothing ever came of that.
You can try using Tumblelog Backup Jammy, just fetch your posts and save the html. :)