Part of the problem, people used to backup to a tape drive and take the tapes off-line; these days most everyone is backing up to hard-drives or storage that stays connected. So, the virus encrypts your backups.

So far the reports are that the virus encrypts only drives that have a letter assigned, so if you backup to a UNC share or a partition that have no letter assigned, your backups should be safe, for now. But there is nothing that prevents a next version of the virus to get to those.

As such, not only do you need to backup, you also need to devise a way of taking the backups offline.