(I didn't opt to randomly choose the middle letters, but instead sorted them, using reverse alphabetical order when the word came out the same. This makes some words insanely difficult--functional and interpretation are impossible to pick out so mangled, and I wrote the entire paragraph before pasting it).
If you thought that was too hard to read, here's the original:
Actually, that is not true. It seems like it ought to be, but that is in large part due to only very slight munging done to the text. If you try harder, and apply a more aggressive transformation to the text, you will find that it is no longer easy to read the text. Especially note how the longer words in this sample are far harder to read than those that are shorter; it is the omnipresence of the small, functional words that lends itself to easy interpretation, and longer words (more heavily mangled) become exceedingly difficult challenges to puzzle out what they were originally intended to represent.
I speedread though so maybe it's because I'm already used to not actually focusing on letters and taking in the gestalt instead. It's kind of like how it's easy to tell the equality of two sets written in different orders up to about size 4, and past that it gets tricky. This is why the classic tick mark system uses 4 ticks and then a slash for the fifth too.
It also includes a link to a handy text jumbler written in AWK: http://www.stevesachs.com/jumbler.cgi