Arun Sirpal shows that Transparent Data Encryption will also lead to encrypted backups:
Ok great let’s check this backup file using a cool tool (XVI32). I really want the contact number of a guy called SQLDOUBLEG because I need his help tuning my SQL Servers so I go looking for a text string, hopefully the phone number will be close by.
As Arun points out, you don’t need TDE to get encrypted backups, but it does the job.