I don't think that spelling amongst the populace at large has become any better or worse since the internets made their first appearance; it's just that we're now subjected to so much more of the crap that people produce because they can easily publish it to a wide audience themselves. When everything was in print, it all went through legions of proofing, copy-editing and rewriting to ensure it was saying what the author/commissioner wanted it to say and reflected professionally on the publisher. Now, any idiot can throw something up and correct it later if someone complains (which is, I suppose, the same process in a roundabout way and with more arguments).
I agree with your point with relation to our presumptions about the intelligence of the writer. I know it's shallow, but I often find myself disregarding a perfectly good argument because someone wrote 'they're' instead of 'their' and finished the sentence with 'lol' rather than punctuation.
On a vaguely related note, the number of people who don't know how to use a thesaurus properly is staggering. I spent half an hour, once, trying to explain to someone that just because a word is listed under what you just looked up in the thesaurus doesn't mean you can swap the damned thing out! Gah. I gave up after that and let him get a D (yes, this was a while ago). Did that have anything to do with the topic? No, I just felt like a rant. Gnash, gnash.
