Secure usage is an important thing, but enforcing a 5 second sleep timer AND printing a warning to STDOUT is a bit much. It is okay to print the warning out to STDOUT because a user has to make a conscious effort to redirect it if he/she wishes to not see it. Having a 5 second sleep that cannot be bypassed is problematic in situations when someone is doing some quick and dirty scripting and it should be removed.
Another way to push people to use the .s3curl file is to offer the option to amend the file using the values provided at the command line, but such an enhancement will need to be the subject of another issue.