Have you heard of bootable media? How about live operating systems like Ubuntu or Knoppix? If someone has physical access to your server and can reboot it and boot to a CD or DVD then you might be in for some real trouble. All of the servers unencrypted data will be readily available to the hacker.
Prevent this by going into your BIOS and setting a BIOS password today (and btw make sure that CD is not listed ahead of the hard drive (or raid array) or the password will be useless as the system will automatically boot to the disk!!)