> I'd guess the idea would be to pass around a copy of <snip>
Diffing your copy of Hamlet.txt against a clean copy would reveal the pattern of spaces (ie, the cyphertext), so the security by obscurity (if any) would be easily defeated.
First, let me say that encoding bits in whitespace is to Steganography what ROT13 is to Cryptography. Neither has a chance of success against any non-incompetent attacker, but they serve well as simple proofs of concept.
Second, you assume there's one canonical Hamlet.txt to compare against (which there's, if Alice was dumb enough to pick whatever is the first available option in Guthenberg.org as her cover message). For a more sophisticated attack, you must consider how many different editions, reprints, etc, have there been of that work over the centuries. For each of those, you must consider how many possible digitalizations can be obtained for different brands and configurations of scanners.
Then, there's the issue that you must do all of this for every large message that Internet users send to each other...
Diffing your copy of Hamlet.txt against a clean copy would reveal the pattern of spaces (ie, the cyphertext), so the security by obscurity (if any) would be easily defeated.