This is not related with font hack -- it can't be. It's impossible.
However, I also noticed some weirdnesses lately. As described in the jbpatch thread, the gear wheel (Unicode symbol) also suddenly stopped displaying properly on my device, and my Kindle behaved completely erratically for some time (wouldn't open ANY book or application, just giving errors). I don't know what is going on here, but something is phishy. Maybe Amazon has declared war on us and is now shipping OTA updates to break our devices