We knew that Left 4 Dead 2's new expansion, "The Passing," paid a graffiti tribute to Dead Rising's Frank West. It looks like the game's creators admire Plants vs. Zombies as well.
Reader Stephen H. sent in this grab - we're trying to get a bigger image - but evidently Crazy Dave is the guy to see if you need an M60. Crazy Dave, of course, is your neighbor (and the armorer/store owner) in PopCap's Plants vs. Zombies.
This too is in the room with a pool table on the second level of "The Passing." Just scan the wall, you'll hit it and many other zombie genre in-jokes.