How to set password on the "media" folder?

As well as users, you can assign users to "groups". The access rights for these groups can then be tuned for specific shares.

In the scenario your describing you'll probably want to allow "guest" access to the share where your media resides over CIFS. This will allow any client on the local network to see and access the files. Then you can put the group that your user belongs to into the "write enabled users / write enabled groups" section and set the default permissions for that share to read-only (at the top). So the PS3 should in theory be able to see and stream content, but not change anything. Other users will then need to supply credentials to "write" changes to that share if you specify individual users, or groups of users if you specify a group which a user belongs to.

if you want to hide the contents of the share from all but specified users, set the default permissions to disabled, but in this scenario i think you won't be able to use guest access, and all clients will need a valid account to access the shares. So this may defeat the purpose your trying to achieve with the PS3.

Don't worry about internet based reading. CIFS permissions only apply to the local network (or readynas remote).

I have a similar situation. Media Player / Windows 7 will utterly destroy any MP3 files album art if Windows is allowed write access to any share with MP3's (or any music media player recognises) in, by replacing / adding awful quality album art and other clutter. To stop this, I access the shares with two sets of credentials, via two methods.

In windows you can only specify one set of credentials for any given source, but you can get around this by accessing a target via hostname or via IP address. Windows treats the two methods as seperate entities. So on those machines where Media Player is installed, only read-only access is granted, and is accessed via \\NasName\Multimedia, and on the machine I need to use to upload the files with I have the same method, and a different username / password and connect via IP address, e.g: \\192.168.0.2\Multimedia which is write enabled.

Take a look at http://sphardy.com/web/permissionsguide (note the Ultra only has User mode so ignore references to Share Mode and Domain Mode).