I've been using NullRiver's MediaLink for a little while, but since the PS3 v2.4x firmware update MediaLink has been encountering network errors while streaming MP4s. I've emailed NullRiver about the issue, but haven't heard anything back yet. I've also tried EyeConnect (didn't work at all with PS3 v2.4x firmware) and Twonky (a little clunky under OS X and it is ~$50)
So this weekend I compiled MediaTomb and it has been working very well so far. There are some good instructions on setting it up here: How-to: Stream media to a PS3 from a Mac
One thing I need to add to the instructions above is that you'll need to install the latest version of XQuartz in order to successfully compile MediaTomb on OS X 10.5.x.
I am having the same network errors and also have gotten no response from NullRiver. If anyone has previous experience with the company, it is normal for them to not respond to support requests? Perhaps they are on vacation?