I was experimenting with backing-up Tumblr to a WordPress blog and I found the following script that puts all your Tumblr posts into an RSS file than you can then import.
This tool will create a WordPress compatible XML file from your Tumblr blog, which you can then save and import into WordPress.
I used it earlier today to import more than 500 Tumblr posts and re-blogs to tumblr. and it worked well. The script basically packages up your Tumblr blog as an RSS feed. After that, use the RSS importer tool in WordPress to import them all. I tried the Tumblr importer for WordPress, but I think my Multi-Site setup was balking it.
After the bulk import I will be using FeedWordPress to syndicate my Tumblr posts at jimgroom.tumblr.com into tumblr. regularly. I am thinking of this as an experiment and backup, but the backup vision is fraught with issues given that most of the media has a tumblr.com URL and is not being imported onto my server (which would be awesome if I could swing that). So, as far as an archive, this is imperfect at best, but I like the idea of having it all on the bava as well, and maybe someone smart will have other ideas for capturing the media.