I can't say enough about backing up. Just as important as backing up your email is, so is backing up your Address Book. It can literally take hours to add in everyone's name, address, phone number, screenname, etc.
I can't afford to go back and re-enter address... especially ones I don't keep in a rolodex on my desk. The address book is my rolodex.. so I need to keep it backed up.

You can do this too -

First, Open 'Address Book'

Next, choose 'Back Up Address Book'

Now you will be prompted with this dialog box - you can change the name to whatever you wish.

Choose a safe place to save it - for this example, I am placing it
on my desktop, but I will move it to a CD-R later.

If you ever need to recover it.. or if you want to copy it to your
laptop, simply choose 'Revert to Address Book Backup'
and select the file you saved.

Easy!
Now -- Go backup your address book!! - You will be glad you did!