Previously, it was only possible to choose a single GPU or all of that type (CUDA or OpenCL).
Now, a toggle button is displayed for every device.
These settings are tied to the PCI Bus ID of the devices, so they're consistent across hardware addition and removal (but not when swapping/moving cards).
From the code perspective, the more important change is that now, the compute device properties are stored in the Addon preferences of the Cycles addon, instead of directly in the User Preferences.
This allows for a cleaner implementation, removing the Cycles C API functions that were called by the RNA code to specify the enum items.
Note that this change is neither backwards- nor forwards-compatible, but since it's only a User Preference no existing files are broken.
Also, I wasn't able to test this completely since I don't have access to any CUDA card right now - I tested with the CPU OpenCL devices of both the AMD and Intel framework, and at least for those two the MultiDevice code worked.