I've said this before, but its worth repeating: just because the Echo will run Android doesn't mean it will run our favorite apps. I think there's a couple/few different ways Android developers lock down their apps so they only run on certain devices. It seems like Netflix runs on just about everything, but the same cannot be said for Hulu and HBO Go. Both are getting better about supporting more devices, but I suspect Ceton is going to have a hard time attracting their attention.
HBO Go might actually be more likely than Hulu Plus. At least HBO Go runs on GoogleTV- there's still no Hulu Plus app for GoogleTV. It's hard to say how relevant that is, given that the Echo will run "regular" Android apps, not GoogleTV Android apps (Which, by the way, is probably going to make navigation with a remote a giant pain. I wonder what Ceton's plans are there.).
I'll also add that I'm not expecting Google Play support. Google licenses their proprietary apps, including the Play store. Ceton would need to get Google's permission to load those apps. Ceton might decide its just easier to just use the Amazon app store. That's probably not a big problem, although I've noticed developers don't always update their apps in the Amazon store at the same time as the Play store. Ceton probably could open up a hole to let us install GApps, which is how the people running custom Android ROMs get those apps, but I wonder if that could create problems for them if they wanted to support copy-protected video in the Android environment (since the usual way to install GApps is to install it on an unlocked device from a custom recovery).