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.
Oh, I've got lots to post about in here...I just never get to it. In the mean time, the McLaughlin Group's 22nd Annual Year-End Awards air last weekend and this coming weekend. If you missed Part 1... The transcript (mclaughlin.com)