Y'all remember Ensgilh 101, right? That's where you keep the first and last letters of a word in place and scramble the rest. It mkaes the wrods odldy rdealbe. G just sent me a follow-up:

"Anidroccg to crad cniyrrag lcitsiugnis planoissefors at an uemannd, utisreviny in Bsitirh Cibmuloa, and crartnoy to the duoibus cmials of the ueticnd rcraeseh, a slpmie, macinahcel ioisrevnn of ianretnl cretcarahs araepps sneiciffut to csufnoe the eadyrevy oekoolnr."

Scrambled internal letters make legible words. Simply inverting the internal letters makes words that are very difficult to recognize. Wierder and wierder.

(The source for all this is reportedly slashdot.org (for the original) and the University of British Columbia's Linguistics department (for the new one). I can't find either of them at either site so I can't verify but figured I should note that in the interest of fairness.)

UPDATE: I hate it when I get got and I definitely got got this time. Inverting the letters has no more effect on readability than scrambling, except that expected characters are not seen in the beginning of the word. The major thing at work here is word familiarity. The "invert" phrase above uses many words that are not common parlance. Yes, we know all of the words but they are not words that we commonly read. The English 101 paragraph used very commonly used words. The second factor is placement in context. In the English 101 example it is very easy to read the words in context based on the words around them as there are many "gimme" words used. That is, words with 5 or less characters that we recognize without actually having to read the word and whose readability is unaffected by misspelling. This second example is built to defeat that skill. They also throw in several misplaced commas to break it into illogical segments and make it even more difficult to read.

Case in point, in correctly spelled form the "invert" paragraph has a Flesch Reading Ease of 0.0 and the "scrambled" one has an Ease of 51. The "invert" uses big words (average of 6 letters per word) while "scrambled" uses small ones (average of 4 letters per word).

We hates the tricksy linguists, my Precioussss...

UPDATE2: MojoMark points out that this is on Snopes.