No. Not at all. They are sent in plaintext. Without SSL, a POST request is just as secure as a GET request. Sure, it may not show up in the URL, but it is not secure in any way.
Will this password be encrypted if there is no SSL?
No, the password will not be encrypted. The only reason it doesn't show up when you are typing it in is just the browser obscuring it so that people nearby cannot see your password in plaintext on your screen. It can be easily revealed by inspecting the DOM or using some javascript like this: javascript:alert($('input[type="password"]').value) .
Is it possible for the post message to get intercepted somehow, say using a packet sniffer, and for the intruder to view the post request's text?
This is absolutely possible, and very easy to do. A packet sniffer does exactly this, and can view POSTed usernames and passwords in plaintext. To someone sniffing HTTP requests, a POST request looks pretty much identical to a GET request, except a POST request has "POST" at the top instead of "GET."
You didn't ask, but yes, post requests can be forged as well.
