tar(1) flags as a blackboard prop, that's nice. I wouldn't consider them that complicated to warrant being a background image of such a nature, though.
I can sympathize. I have 4-5 languages I write once a month or so. Each has a different "if" syntax, and a different "for" syntax. Pretty soon I'm going to cave and write myself a little reference note like that.
(Yeah, yeah, I'm sure any real ninjapirate worth his salt can perfectly recall every syntax rule of every language he's ever written)
Slightly off topic, when I'm pair programming in ruby, I often say to the other person: "No you can't break the line there!" The other person rightly shows me that it works fine. Then they say, "Why did you think you can't do that?" My answer is always, "OMG... what kind of insane parser have they written to allow that to work".