Logic Bombs are more like macros. Say a kiddie wants to set up my computer so that if you do not hit the right keys in the right order (let's say crl-alt-1) before they login in as root the system automaticaly begins to shred files then format while a nice little gui screen sits out front saying "now logging in" or something like that. This would keep law enforcment from seeing what they have on their HDD. (good thing most kiddies are not that smart) also the data would actualy have to be shredded not just unmarked or forensic software would still be able to read the information.
Of course you could impliment this in to a virus. Like Palemoon said if you set this same thing to happen whith ctrl-alt-del you'd have a little problem on your hands.