Just thought I'd add a few notes.
I've heard of there being 4 cheats for a while, but looking at the code has confirmed there are for sure, without a doubt, exactly 4 cheats. What these cheats do is also now well know. As for what text strings activate each one, there are variants of them all, not just the fourth cheat. For instance, if you were to replace a final period with a space (or any other letter) it would likely still activate the same cheat. In fact, the way it was done, you could have typed a random innocent sentence and accidentally activated a cheat. Granted, assuming an equal probability distribution, your chances are about 1 in a billion, but it was possible.
Also, I'd like to point out that we don't actually know what the fourth cheat code is (officially). We know of three text strings that activated it, but none of them were really "correct". They were all just those random collisions I mentioned above that we had about a 1 in a billion chance of randomly finding. (Granted, the strings weren't exectly randomly chosen, and their method of hashing creates some degree of clustering of solutions, which is why we found 3 all at once). So, in short, we have a number of text strings to ativate any given cheat, but only the first three are really "official" cheats. (All known variants of the 4th are grammatical nonsense, so there is no way they could really be right, but they do still work).
But yeah, all of those cheats are now disabled. Although I have to admit, I'm still very curious as to what that 4th cheat was really supposed to be.