Take a look at r1soft's (commercial) product.
This runs an agent that continually backs up your data, and has a very easy restore-entire-server-to-bare-metal feature. If it was free it'd be the perfect backup solution :)
But that's a solution for entire servers. If you want to backup a subversion repository, there's only 1 thing you need - svnsync. Its easy to use it - create a repo on the backup server, initialise it, then sync it. After the initial copy is complete, you can run svnsync regularly to copy all incremental changes over, so its fast. In fact, its so fast that most people place the command to run it in their post-commit and post-revprop-change hooks. So the backup server is always completely up to date. (I run it hourly, but I'm just perverse like that).
You don't need to be logged in, and you can sync from the backup server - to pull changes from the live repo, or from the subversion server - to push changes to the backup. If the backup server is not present, the next time svnsync runs it will grab all the changes, so you don't have to worry at all.
You just need a server to backup to - any of the commercial subversion hosts will do for this, a backup repo is exactly the same as an other subversion repo, and to use it if the live server dies, just requires you to "svn relocate" to the backup repo url.
Google 'subversion hosting' for quite a few online hosts.