[robmurrer]

If I look under your doormat, and there is a key, and I use it to open your front door...

[josteink]

Rather if you leave a (possibly classified) document under your doormat, am I a criminal if I find them and read them?

[rapala]

Depends on the document and jurisdiction. If I remember correctly, some levels of military classifications here in Finland require you to not read the document and return it to the officials. Of course the one who left the document would also get reprimanded at least.

Using someones password without permission is as illegal whether you shoulder surfed it, cracked it or red it from a post-it note.

[nitrogen]

A house's front door implies an expectation of privacy. A web server implies an expectation of public access.