First, from GOOD: Meh Makes Its Mark
Good overview, including a bit of musing on what makes a "real word" (it's the source of the quote I used to title this post) and why meh has gained enough popularity that it makes its debut in a dictionary of British English, despite having American origins -- apparently it first appeared on The Simpsons. Author Mark Peters also lists several other Simpsons-isms that have made their way into common usage -- "d'oh", of course, and "cromulent", but also "yoink" (another word that I've been using for years without realizing its Simpsonic origins).
The other is from Word Routes, the language blog at Visual Thesaurus (one of the coolest language tools on the Internet): Mailbag Friday: "Meh"
More on whether or not meh is a "real word" or just a "sound effect". The trouble, as author Ben Zimmer sees it, is that meh is an interjection, and lexicographers tend to immediately see them as suspect when compared to other parts of speech. Most interjections are onomatopoetic (sound like what they mean) and operate outside the traditional rules of grammar. Zimmer lists the 163 interjections found in Merriam-Webster Collegiate dictionary, and notes that many of them ("aw, bah, eek, eh...") don't feel like "real words" either. His conclusion is that meh is a "real word" because people use it like one and, descriptivist that I am, I tend to agree.
As a side note, meh was selected from a pool of neologisms submitted by the public. One of the other finalists was "frenemy", meaning an enemy disguised as a friend. Although not as broadly useful as meh, I'm pretty fond of that particular coinage -- a single word, used to convey a complex concept but immediately obvious in its meaning. If that's not what a new word is for, then what is? (Although my browser's spellcheck doesn't recognize either of them yet. But give it time.)