I suspect it dates back to the days of Usenet and the restrictions of 7-bit ascii.
With no pretty HTML or faux-HTML tags to use to make with the bold and the underline and the pretty colors and the hey-hey, you have to find a distinctive way to emphasize.
I actually seem to recall something about l33tsp34k being the origin of the tildebang, but that's just heresy at this point (although I'd find it fitting that the IWC took a page out of the book of B1FF~!).
Then I can write a washing bill in Babylonic cuneiform, And tell you every detail of Caractacus's uniform; In short, in matters vegetable, animal, and mineral, I am the very model of a modern Major-General.
I searched google far and wide to try to find the use of tildebang outside the IWC, but could find nothing but a Mac-fanatic who likes to torment the (former?) publicist of Ambrosia Software by calling him Jason WHONG~!
Given that buddy with the site has a picture of himself on the web with a viking hat on, it's very, very possible he reads the IWC. In fact, his writing style seems to have taken more than a page or two from Dean.
In closing, I was way wrong. All hail DEAN~! Innovator of Punctuation.
"Web 2.0" sounds more like a C average. Actually, that may be apropos given some of the "website experiences" some of these places give you. (This is not clever enough for me to remember, but probably too clever for me to write down....