First of all, I don't tend to trust everything I do to wikipedia, it can sometimes be unreliable.
Well but it's better than having no reference whatsoever... Also, do you really think that the wikipedia entry is wrong here about "to google" being in a dictionary? If not, then your statement is a red herring.
However, in this case, this is not a brand name being entered into the dictionary, this is a brand name becoming slang, becoming a real word and entering the dictionary, what I described in fact.
I disagree about your definition of slang (but no I'm not going to quote wikipedia again, especially as in such a case it's more vague indeed), but okay, let's assume everything that's not in a dictionary is slang. You still haven't replied to my point why making a new (slang) word from a brand name should be "incorrect", grammatically or otherwise. Again, if you accept that LEGOs in this usage is a new word (because it does exactly *not* refer to the company, but to the bricks), slang or otherwise, why should it be incorrect? "To google" sounds pretty awkward, too, but hey that's the way people use it, and now it's an "official" word.
The man who first wrote the dictionary died before it was finished. I can't remember the exact times but I think it took over 10 years, and it defined the English language for the first time ever, completely, with every word that was used throughout the country. People still had words and spoke before the first dictionary, he did not sit down and make up a list of words. Grammar existed before the dictionary, obviously, as well as the words, he merely set out to document and describe each one of them.
So following your argument, all language before the first dictionary was slang? And what about that grammar that existed before the first dictionary, who said what is right or wrong?
Really, right or wrong is usually understood in terms of how people commonly use language, and apparently, dictionaries follow exactly that reasoning when introducing words like "to google" or spam email or whatever.
So, your definition would be everything that uses words that are not in the dictionary (yet) is grammatically wrong? Or would you say that LEGOs is not a word at all, slang or otherwise? Despite of the fact that people use it?
Edit: You know what, I think the problem would be solved, from your point of view, by refraining from calling things "incorrect" when talking about mild slang...?
Edit2: you know what, where's the guy who started this whole discussion anyway. I'd say we find a compromise by throwing him into a lava pit.