This wasn't about unplugging and plugging a device in. It was about crashes. If the BFL unit fails at some point (as is happening with one of mine), it used to just die and be dead forever. Now, it goes into a disabled state and you can attempt to restart it from the API without having to completely restart cgminer.
I don't think anything has changed with hotplug support (i.e. it's still not supported).
From your description I assume you are running Linux. I'm on Windows, where cgminer 2.3.6 quits to the command prompt if a BFL dies. I'll have to do some testing of 2.4.0 to characterize its new behavior before general deployment.
Currently I launch 2.3.6 from a (Windows) batch file containing a loop; if a BFL unit dies, 2.3.6 exists and the batch file simply restarts it automatically. Works well for me, which is why I hesitate to jump onto 2.4.0 without first understanding its changed behavior.
Regarding cgminer's API, anyone have experience using it under Windows?