Yes, people can't spell or construct proper (or even nearly-proper) sentences when online. Part of it may be the "it's just an e-mail" (or chat session, or HTML form) mindset, but I don't think that's all of it. I've seen people write papers for class that have rather glaring errors. I've even seen a professional research paper with some obvious misuses. I don't recall seeing this before the mass-entry onto the web. Am I saying the web's responsible for the bad spelling of today's youth? No, I'm not so silly as to make such a comment. What I am saying, however, is:
It does affect you.
I should likely speak for myself: It affects me. For example, I had to think for a bit as to whether 'affect' or 'effect' was the proper usage. There are probably far too many commas in this post. I've misspelled something, I just know it. I'm not concise enough.
The rub is that I know I used to be a better writer than this. I've maintained a homepage (not the link above) for five years, and have updated it nearly weekly for that time. While my writing was certainly worse when I started -- for example, there were plenty of tense shifts (**shudder**) -- I am making mistakes today that I wouldn't have made a year or two ago.
Most of the people I communicate with online are used to the medium, and so tend to spell correctly and use good sentence structure. Increasingly though, I have been communicating with friends who aren't so computer savvy, and problems abound in their writings. Obviously, I'm not going to flame my mother for poor use of grammar, so I learn to simply tolerate it as I can just figure out what they're trying to say. This leads to my brain automatically skipping over any malformed portion of a work - including my own work. Lately I've had to conciously think about spelling (like the spelling of consciously), punctuation (it's, its, and its'), and grammar (I must have made countless mistakes in this post alone, even while being sure I was correct). The more I'm around sub-par writing, the more it changes my own writing ability.
Needless to say, I'm hoping this change is only temporary. I would not appreciate unlearning several years of elementary school training. If K5 makes me repeat the third grade, I'll be quite upset. :)