As in everything: It depends.
I don't know this story beyond what two people have just said. But, I know that if I commit to something, I expect my co-founder to also be committed to it (within the bounds of what we've agreed upon). Working on other projects now and then is fine and normal...it's what hackers do. But, if you've committed to build a business with someone, it's not really the idea that makes the business. It's the combined effort and execution of the team. So, splitting your time between building your "private business" and your "co-founded business", and then taking off when the private business has some bit of success, is probably not what an honest business person would do. If you don't believe in the co-founded business idea, anymore, you talk it over with your co-founder, and you try to fix the problem.
But, shit happens. If you give up your equity in the co-founded startup, apologize for flaking, etc. then I guess you've done the best you can. I wouldn't expect the co-founder to ever be willing to do business with you again, but it's probably not what I would call "treacherous", either. I think the story of Evan Williams and Blogger (and Odeo/Twitter, for that matter, maybe even moreso) would sound kind of similar when told by different involved parties, and we don't consider Evan treacherous.
Then again, we are talking about one week later. He had to know that he had other irons in the fire and probably wouldn't be involved in the company much longer. So, yeah, this actually does sound kinda nasty.