To me, an indeterminate spinner says very clearly: no idea how long this may take. Progress bars provide more information, but might be at least as misleading as a spinner. My Netflix app typically loads a movie like this: 0% ... 34% ... 99% ... 99% ... 99% ... 99% ... 99% ... play, although sometimes it does: 0% ... 1% ... 2% ... 2% ... 54% ... play. Fortunately, the netflix progress indicator keeps spinning, so you sort of know that it's to be expected. If it were an unanimated progress bar, I would start suspecting an error when it gets stuck (as it routinely does).