This is similar to rsync's --dry-run option, in that it runs through everything to show you what it WOULD do, but doesn't actually make any changes to the filesystem.
From the astyle -h:
Perform a trial run with no changes made (useful for checking if formatted).
Reason for patch:
I am using astyle to automatically format code on a mercurial commit hook, but in the main repository I also want to deny pushes that contain code not formatted by astyle, with the --dry-run option I can simply grep the output of astyle for '^Formatted" to see if things any files haven't been formatted according to our astyle configuration. This allows astyle to work somewhat like checkstyle.
Hopefully this can be accepted into the main branch fairly soon, as it's helpful and a pretty trivial change. If you have any problems with it or want anything else changed let me know.