There is virtually no cost associated with issuing a certificate. The fact that they are nevertheless prohibitively expensive for most private domains is partly responsible for the failure to adopt SSL on a broad scale.
Otherwise the browser will display a big red warning message.
While it is true that this warning is intended to protect users again man-in-the-middle attacks and other methods that redirect traffic away from the original source, it also prevents people with absolutely valid (but free) certificates from offering perfectly good encryption on their sites. This warning is misguided and does more to prevent the secure use of the web than anything else.
The remedies would be simple, but I guess commercial reasons prevent them from being adopted at the expense of everyday users.
