Now I'm testing out how fast the update gets added to Twitter. I'm pinging http://pubsubhubbub.appspot.com/ with curl when I save the new post, so in theory it should route immediately. It seems to work already for the hosted feed updates (my last post went through really fast), so now we'll see if it makes it through as quickly to Twitter.
By the way, there's a bug right now (which is being fixed) where if you have rel="alternate" in your Atom feed links (like I did), Feedburner would barf on it. So I took those out and poof, it seems to be working.
Update: The feed has been updated, but Twitter took a while... I thought it was broken again at first. Maybe even mentioning that attribute in the body of my post hosed it? :-)