In the latest release of Raspbian a new addition to security has been made.
Now there will be no SSH server running unless a file exists in the /boot partition. This file can be empty and is just called ssh.
If you are creating an SD card on a Linux machine you can mount the FAT32 boot partition and create the file in there by issueing the command:
$ touch ssh
This will create a file of zero length.
I can't help thinking this is a bit ridiculous. Especially as the default pi user still has the ability to use sudo without having to enter a password, making pi effectively a root user.