Refer to http://blog.whatwg.org/reverse-ordered-lists and http://www.whatwg.org/specs/web-apps/current-work/#ol.
As I mentioned in my post to the blog, I believe the 'reversed' attribute of ordered lists should become 'order' instead. With all the hype of HTML5 not breaking in previous browsers, you would think we should worry about it not breaking (or having to hack our way around it) in the future. We may not see a need for any value for 'order' other than ascending or descending, but why limit it to those two choices? We didn't see any possible choices other than 'checked' and 'not checked' for form check boxes in the past, but then people started using 'check all' boxes and that required 'checked', 'not checked', and 'some checked'. Due to someone choosing to use 'checked' as an attribute instead of a value, it made it impossible to do this without hacking it. So I believe HTML should stop using values as attributes and allow for expansion in the future.
Also, will search engines recognize ordered lists with a descending (reverse) order? It may not make much of a difference in small lists, but in a countdown from 100 to 1, it can make quite a big difference.