NetInfo Manager disappeared in OS X 10.5, along with all my automount SMB shares from my Buffalo Linkstation. After poking around the net for documentation on the unix automounter, I came up with this solution to get my automounts going again:
According to /etc/auto_master, the /etc/fstab file is now used to dynamically mount shares under /Network/Servers. So you basically just need to transfer the stuff you used to have under Mounts in NetInfo Manager to your /etc/fstab file; mine looks like this:
excalibur:/photos x url net,automounted,url==cifs://guest:@excalibur/photos 0 0
excalibur:/videos x url net,automounted,url==cifs://guest:@excalibur/videos 0 0
Translating, the above shows that I have three Samba shares called music, photos, and videos. They are all on a server called excalibur, and i'm accessing them using the username guest with no password. The second item in the command is x for each line. That's generally the mount point, but because of the way the automounter is configured, that entry is ignored. The mounts will always be placed in /Network/Servers.
[robg adds: I haven't tested this one.]

