I'll second this. It's commonly used in several blogs that I have (occasionally) followed, and I like the sort of tongue-in-cheek tone its use allows. I've had several occasions recently where I really wished that it were allowed.

I've heard that the <s> and <strike> tags are being depreciated in newer versions of the HTML standard, but I think the <del> tag does something similar. It might also be able to be done in a supported way in a CSS style definition. Assuming it's technically feasible, I vote it be included!