Backing up your Nexus One without root
Rooting your phone is a pain, especially if you have all the latest updates installed so that the known exploits don't work. But the only reason I wanted to root my Nexus One was to make a full backup so that I could restore it after giving CyanogenMod a try. You can create a full backup without root privileges. Note that this very likely requires an unlocked bootloader (I can't verify as mine is already unlocked). This is a quick howto and as such will presume you have a level of expertise.
If you haven't already done so...
Download fastboot for your platform from HTC.
Shut down your device and hold the trackball button down while you boot up again. This will put the device in fastboot mode (the one with three skateboarding androids). Plug in via USB.
This will delete your data! Do some other kinds of backups and sync your data w/ Google first!
fastboot oem unlock
Follow the prompts on screen to unlock your bootloader and technically void your warranty.
Now on to making a full backup...
Ensure you have enough space on your microSD card (my backup took around 300MB).
Download a recovery image that does backups. Make sure you get one that's correct for your device (one with passion in the name for the Nexus One). Checksum it to make sure it downloaded correctly. Get into the bootloader again.
fastboot boot recovery.img
This boots the custom recovery without flashing it. When it's done booting use the menu it shows you to do a full Nandroid backup and then reboot. It will take a couple minutes.
When you want to restore your backup repeat this procedure but select restore instead of backup in the recovery menu. Restore takes longer than backup, be patient.