Arq 4.8 is now available, and it includes support for backing up to your own Dropbox account!
If you already have a Dropbox with 1TB of space, now you can use that space for your Arq backups. The Arq backups go into the folder /Apps/Arq in your Dropbox account.
I've always wanted to try Arq and use the space I have in my Dropbox account. The latest Arq adds support for Dropbox backups and it even lets you combine multiple destinations (such as Dropbox + Google Drive or Dropbox + Amazon S3) to have specific files in locations you choose. Version 4.8 is a free update for existing customers.