Windows 7 has taken Vista's media sharing and streaming capabilities and really put the shine on them, take a look at this post for some great in-depth coverage.
Reading about these new features I was excited to give them a try. Like a lot of people who have large media libraries I don't keep all of my media under my user profile directories (My Pictures, My Music, My Videos), I instead keep them on a large partition on a different (larger) hard drive.
After enabling streaming for network devices in WMP 12 I right-clicked on the Videos library to select Manage Videos library and simply added my D:\Video folder. Then pulling up the media server on the PS3 I am able to see the couple of sample videos in the My Videos folder but nothing from D:\Video.
I thought that the problem was that WMP or the PS3 were not refreshing the list of available files, so after messing around with trying to refresh the data for a bit I was not getting any further along. As an aside, I've found that WMP does take it's sweet time to refresh the media list, and easy way to force this is to go into the Service control panel and restart the Windows Media Player Network Sharing Service this seems to force a refresh of the media that is shown to players.
Well, if it isn't a refreshing issue it must be a rights issue, looking at the security rights on the My Videos folder I did find one difference from my D:\Video folder. The difference was one group named Home Users that had rights to My Videos. To give the group rights, right click on D:\Video folder, choose properties, select the security tab, and choose the edit button. Click the add button and add the Home Users group, the rights default to read/execute with will work just fine so click OK to save the changes.
After setting the rights on the directory and restarted the Windows Media Player Network Sharing Service I pulled up the videos folder on the PS3 and the shared videos in the D:\Video were immediately playable.
The new streaming and sharing features in Windows 7 are really above and beyond the ones in Vista, I'm hoping that in a future update that the PS3 and XBox360 will get updates to support the new Play To feature in Windows 7.