This according to Dan Steinberg, who noticed it on the Wizards web site. A team source has confirmed the news, so I think it's pretty much official.
↵Arenas, of course, wore number zero throughout his NBA career before deciding to switch to No. 6. However, he was reportedly considering a switch to No. 13, according to Truth About It. That would have been a homage to his mother, who died on March 13, but Arenas has evidently decided on No. 9.
↵This opens a whole new can of worms, since Yi Jianlian wore No. 9 as a member of the Nets last year. Considering how many people are down on Yi's development thus far, maybe it's more appropriate for him to be the one to wear No. 0.
↵