Dan at Dan's Deals reported on his experience renewing his passport online. If you have ever renewed a passport, you know that it can take several weeks or even months depending on processing times and whether or not you pay for express processing, but Dan was able to get his new passport in just over a week. This is a great tip to keep in mind. There is an interest form you can fill out to be a part of the test, but I don't know if they are still accepting submissions for that.
Dan's post caught my eye because child passports are only good for five years and my older son is 4 years old, so I thought that this may become relevant for me in the not-so- distant future. It's not much for the time savings there.
One cool thing is that you don't have to send in your old passport to have it renewed online. It isn't clear whether the old passport loses validity at some point during the renewal process, but there are no holes and no being without a passport during the renewal period. There are more details in the previous Dan's Deals post.
I have run down to the wire before. Same-day service is possible if you have a near-term international trip and I did, but at my last renewal I had to drive 3 hours to New York City. I had to be at the office by noon to get service the same day, but I was able to get there in time. That happens on an appointment basis.
If you're interested in online renewal, you should read both Dan's Deals posts and the interest list sign-up page, as it may take several weeks before you're eligible to renew online.