Great article about adjusting /etc/sudoers to let rsync be executed as sudo without a password:

http://occy.net/node/135

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Using visudo -- edit the following file: /etc/sudoers
USERNAME ALL = NOPASSWD: /path/to/command1,/path/to/command2

The problem is that if you do not put the USERNAME ALL =... command at the bottom, then the %admin ALL=(ALL) ALL statement that comes with Ubuntu will override it and force you to enter a password. Negating what you put in above. The simple solution is, put your statement at the very bottom of your sudoers file.