As has been noted above several times, these two filesystems--/net and /home--are related to the new auto-mount daemon introduced in 10.5. I haven't seen anyone mention what their primary purpose is, and that is to provide mountpoints for clients managed by OS X Server. If your home folder lives on the server (and syncs to your local machine), it will be placed under /home, which is traditionally where Unix-like systems store home folders (instead of OS X's usual /Users). And your auto-mounted sharepoints will live under /net, instead of the usual place in OS X--/Volumes.
If you start up your non-managed Mac OS X 10.5 or 10.6 machines in verbose mode (cmd-V at startup), you will notice on shutdown the following two entries on the console screen:
Unmounting of /home failed
Unmounting of /net failed
This is nothing to be alarmed about; the unmounting failed because you never mounted anything there in the first place, because you are not a managed client. Some folks I've seen on the 'Net have wrongfully assumed these entries were due to a bug in Apple's code.
