I was interested to see the headline "LOL -- 'Webspeak' invades Oxford dictionary" on CNN this week. The article is little more than a blurb about some new additions to the Oxford American Dictionary, but I was struck by the first line: "Are years of e-mails, text messaging and status updates finally affecting the written word?" When I read that I did a bit of a double-take, as you might be doing right now. "Hold on a sec," you might think, "aren't ALL of those things written words?" This usage takes to the extreme the idea that "the written word" as a set phrase is somehow not compositional; it doesn't literally mean "a corpus of written materials in contemporary usage", but rather some lofty edifice culled from esteemed writers and curmudgeonly literary critics. While I acknowledge that "the written word" is a semi-idiom in many dialects of American English, I would never use it quite as idiomatically as in this article -- literally juxtaposing a huge corpus of written material with the ethereal ideal of "the written word".
Despite this opening line, the article isn't critical at all of this move by the OAD. The author in fact notes that "It is nice to see Oxford attempting to get with the times" by including expressions that many of us see every day. Lexicographers are often remarkably descriptive, despite the tendency for prescription among those who use their products regularly. However, the author does fear that this will make difficult times for English teachers, as students back up their usage of TTYL and LMAO in academic writing with dictionary citations. I can certainly see English teachers cowering in terror, even though this seems to me ridiculous. As long as we talk about what is appropriate rather than correct, there's no need to fear descriptivism. For instance, I rail against those who teach that it is "incorrect" to use "which" in restrictive relative clauses, or that it is "ungrammatical" to use double modals. On the other hand, in some contexts there are reasons for teaching that it is inappropriate to use these in academic papers (although frankly I'm always against the claim that "which" should be only used for nonrestrictive relative clauses).
The problem with the absolutist view of English is that it isn't absolute. If you try to teach students that "which" should only be used for nonrestrictive relative clauses, you don't have anywhere to turn. The dictionary won't tell you this, esteemed authors don't show this usage, even the venerable old Strunk didn't keep his whichs and thats complementary (though when White came along he added the rule and edited all of Strunk's examples to make them fit the rule). Too often what people think of as "correct" grammar is simply bits and pieces of inconsistent jargon they've internalized from many different, often conflicting, sources. What students need to be taught is that academic writing is a formal style with strict rules. It's not that double modals are wrong, it's that double modals are frowned upon in academic writing. And that's a reason not to use them in such contexts if you want to get a job.