Google has secretly started blocking access to its apps on uncertified devices whose firmware was built after March 16th. If you're affected, you'll get a warning that a device is "not certified" and can't sign into a Google account, according to engadget. Because Android is open source, it gives device makers the freedom to customize the software as they please.
However, in order to ensure a level of consistency between Android devices, Google has cleverly leveraged their suite of popular Google apps and services to mandate that device makers follow a specific set of rules under the Compatibility Definition Document (CDD).
A device maker must follow the CDD to pass the Compatibility Test Suite (CTS) so they can be allowed to pre-load their devices with Google apps and services. Previously, it was possible for uncertified devices to still make use of Google apps (shortened as ‘Gapps’ by the community), but now it appears that Google is cracking down—while still allowing custom ROM users to use Gapps!
This won't prevent you from loading ROMs, but you'll have to register your device IDs on a white list every time you undergo a factory reset -- when there's a 100-ID limit, you could run into problems if you're routinely wiping your phone to install new firmware.
The company is already quite clear about the rationale, however. Vendors officially need to get certification to load Google apps (Android's licensing model is built around this), but some of them ignore this requirement and either load the apps regardless or encourage you to download them yourself.
This new block theoretically pushes those less-than-scrupulous brands to clean up their acts by either obtaining certification or sticking to an AOSP version of Android, where they'd depend on third-party apps.
The problem, of course, is that it leaves custom ROM users caught in the crossfire. They may have to be more selective about when they install ROMs and skip minor updates. It's far from the end of custom firmware, but the era of anything-goes firmware (on phones that allowed custom ROMs in the first place, that is) might be winding down.
