There's been a lot of speculation about just how Nexus-like the Galaxy S4 and HTC One Google Play edition phones will be, particularly from a technical / software update standpoint. Now, we have some relatively concrete information that sheds light on these issues.
First and foremost, Google will not directly handle software updates for Google Play edition devices. This has been reported as true, false, and generally disputed quite a lot in the lead-up to the launch. We know for a fact now (thanks to Anandtech) that while Google will supply the necessary Android builds to OEMs, the OEMs will be responsible for maintenance of kernels and all the various drivers and firmware pieces necessary for their specific device. The result is that Google gives the OEMs the Android build, and it's then up to the OEMs to make that build work on their Google Play edition device and perform the act of actually updating the phone. Google has issued an official statement that works very hard to not expressly deny that they won't be handling the updates (Droid-life):
On background, we will be working very closely with Samsung and HTC on future software updates for these devices and they will receive software updates shortly after a new version is released.
What's this mean for you? It means that at the end of the day, the onus to update these phones will still be on Samsung and HTC. They will be able to circumvent the carrier certification runaround, and will in theory have much less work to do in maintaining these devices, but the responsibility still falls to them, not Google. This could mean any number of things. It could mean these phones will get updates within days of their Nexus counterparts. But more likely weeks, and possibly even months depending how much attention these phones are actually given by HTC and Samsung. I wouldn't say months is what we should expect, but I'm also saying such a gap is entirely within the realm of possibility. You have to remember, these phones will sell in miniscule quantities compared to their skinned brethren, and that means they aren't as high up on the value chain for software update attention. While it's certainly worth being optimistic about the update process, there's still plenty of reason to temper those expectations until we see these devices receive their first major Android release update.
The second thing we've learned is that Google will not host factory OS images for Google Play edition devices (see JBQ's post on this topic here). This will be up to Samsung and HTC, if they choose to do so. Google will also not be hosting any proprietary binaries (essentially, drivers) for these phones (again, see JBQ). Release of any binaries is, once again, up to Samsung and HTC. Taken together, all this information - as you may already have guessed long ago - means there is very little chance we'll ever see these phones supported in AOSP. It's not impossible, but it is extremely unlikely.
So there you have it: that's basically the nuts and bolts of what doesn't make the Google Play edition HTC One and Galaxy S4 Nexuses. Does this put a damper on the situation, or is it immaterial? Let us know in the comments.