This would seem to be an overall nice feature to be built-in.
Two levels would be best:
1) ability to require a password for a PS3 to connect to the media server overall.
2) extra bonus... ability to set up password for different folders.
#1 I've seen in other media servers, and would be a really nice feature to keep someone who might get on the network from access. (my WiFi is pretty locked down... but one never knows)
#2 would just be really cool, though I'd imagine somewhat complicated.
The above reference to a solution seems to be kind a Windows-only hack using Windows rather than the server to accomplish it. I'd think being built right into the server would be the better way.