Headline News: SD cards have NOT stopped working - but some methods that apps used to use will no longer work.
Back when Honeycomb was released, Google introduced a new method of accessing SD cards, which was largely ignored at the time, but is now being implemented by Samsung (and maybe by other manufacturers who have devices with MicroSD card slots). There's a long Android Police article about it, but here's the TL;DR version.
- Every app still has read access to every file on your SD card.
- Every app can still write to your SD card, but it will have to use one of the new methods. Simply having write access to the card won't work any more.
- You do NOT need to root your device to regain write access to your SD card.