Just about every major blogging tool I know has some sort of "export" capability and that's exactly what you want to do with Movable Type too. When you log in as administrator one of the options on the left-side menu should be just what you seek:

Go ahead and click on it. You'll see lots of different options, but they're for importing blog data from a file (an export file, of course). Instead, click on the "Export Entries" tab and you'll see this:

Note the warning, but don't be too paranoid. In my experience, the export does give you all the entries and all the comments, which are 98% of what I need so I can sleep at night, less worried about the hackers and crazyfolk of the blogosphere. :-)

Ready? Click on "Export Entries From {your blog}" and the download window of your Web browser should pop up and show you that a ".txt" file is being downloaded:

Once it's done, you have your backup. It's not necessarily human-readable, but other MT blogs can import the data (as can a variety of other blogs, since as far as I know, the import/export data format is consistent across platforms). My export, by the way, is 6.1MB, representing 6,355,534 letters, 988,862 words and 112,185 lines of text.

Purely by way of contrast, the King James Bible has 783,137 words and 3,566,480 letters. Draw your own conclusion on the relative importance of these two tomes. :-)