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The easiest approach might be to simply tell OmniFocus to write its backups to a directory in your Dropbox. Then you don't have to worry about it, and you'll get a backup at least once per day. You will have to prune your backups occasionally, though, as OmniFocus just keeps writing them...

Otherwise, File->Export... choosing OmniFocus Backup Document as the file format and saving to your Dropbox should do what you need on a one-shot basis.