When I was going to conferences, my laptop stickers were a public display of "these are technologies that I use and you can strike up a conversation with me about them." To an extent, a resume that you can glance at from across the room.
It's a bit of a statement for what you're trying to communicate with that lid - professional experience, political statements, personal "this is neat"...
And part of this is a for me the lid of the laptop is something that I'd need to be able to be comfortable with displaying in front of a CxO without worry about if they may be offended or not (though perl might be offensive to some).