Hey - I did a web search and the most common conclusion is that if your device is working well, don't worry about how full it is. Another proposed that Apple would build in space you couldn't fill if the blank space was necessary for device operation. (They don't do that with the computers, but then iOS ≠ Mac OS.) Nothing really definitive anywhere. Does anyone know anything for sure?
I can tell you, however, that my next iPad will have a 64G capacity! I'm freeing up a little more space on my iPad by compressing the music.
