"...the dots died quite suddenly in (I would say) the early nineties..."
Ah yes, the advent of the technological age: birth of widespread human laziness. I would say that just because something is common does not make it right or correct (and yes I use those words separately).

Warsaw Will: while I agree that "Be that true" does sound quite incorrect, what about the phrase "Be that as it may..." which I hear often?

Makes me wonder...
By the time he arrived at school, the lesson had finished.
Suggesting that he is late, the lesson is completely over, and he has missed it.
He arrived at school by the time the lesson had finished.
Seemingly indicating (to me) that his goal was to get there by then at the latest, and he succeeded by arriving when (or before) the lesson was over. Similar to 'Be home by the time dinner is ready."

Are these correct understandings of the different placement?

Not an attempt to incite wrath, but a genuine query: wouldn't that be "He looked starved"? or can "He looked starving." be correct?

Oh wow, fun times!
First, I'll address Becky (btw, AnWulf's o button does work as you can see the o in other words... and it also bugs me that Anwulf types "yu"):
Anyways, to my point. Re-read J Anthony Carter's first post. It describes the answer most simply. When considering the original sentence, 'language rules set in stone' is one object on a list of many objects. Regardless of whether that object has more sub-objects, the correct word can only be 'is'. It doesn't matter if you change 'set in stone' to whatever else you want; 'language rules' is a singular noun in this context (and context is everything). Changing it's adjective does nothing to the requirements of the verb.
After re-reading your last post, it seems that the idea you're having trouble grasping is that 'language rules' can be a non-plural noun. I assure you, in this case it is.

Moving on to Hairy Scot's post: As a standalone sentence, "Language rules set in stone make me shudder" does not use 'makes' because the subject is given to be a plural group. There is no context given to show it as a singular object of a larger group (and remember, context is everything). As for your other examples, the sentences you give each have a singular noun as their subject. Both crowd and team are non-plural. The individuals within those groups are plural, but the group is singular.

*Or you could substitute 'horizon' with 'tangent of the Earth'.

I may be an English nazi with my friends, but math has always been my favorite subject ;) Thus, I use the word 'perpendicular' to mean anything at a right angle to the object I'm describing.
I would think 'plumb' is equally suitable to your task, and probably more widely recognized as being reserved for 'perpendicular to the horizon'.

A somewhat interesting thought occurred to me just now. If two walls are to be truly perpendicular in the sense of your favored meaning of the word, then they cannot be parallel to each other, for the earth is round, and the horizon from each point would be slightly different (that is, if we can use the idea that another way to explain perpendicular would be to say that it must coincide with a radial line directly from the center of the Earth). Sorry, rambling ;)

Is it as difficult to look past the inane posts for others as well? Or is it just me? ;)
"I'm" cannot be a complete sentence, because in the case of the contraction, the 'am' used in 'I'm' is an auxiliary verb, requiring another part; it is not the state-of-being meaning of the word.

Porsche, I congratulate and admire pretty much everything you've said here :)

For me, it's far less important to discover the 'shortest sentence in the English language' (especially since English itself is not only a confusing hodge-podge of adopting everyone else's language but also inherently adaptive to whatever one needs it to be) than it is to figure out what the underlying rules are and better understand how to use them.