Jailbreaking your iDevice gives you the ability to customize it from head to toe. You can change the complete look and feel of the device by editing icons and installing extensions. Another area you can personalize through jailbreaking is the boot screen.
The default boot logo is of course the infamous Apple logo, but with a little tweaking you can change it to virtually whatever you want. In some of the more recent jailbreaks though, the ability to customize the boot logo was compromised...