Hey, don't know if this thread is still active but here's what you want to do, you want to create a login hook (if you don't know what it is Google it! There's ton of info out there about it including a gui called iHook) which is a shell script that run at login. The script itself will look something like this:
#!/bin/bash
mkdir /Volumes/<Share Name>
/sbin/mount -t smb
smb://username:password@172.0.0.0/<Share Name> /Volumes/<Share Name>
This will mount the share with the given user name and password (for more information on the mount command type 'man mount' in the command line) at the mount point '/Volumes/<Share Name>' (which can actualy be anywhere you want but i like to keep OS conventions). Note, you probably want to set user name to 'guest' as there is no (thank god) variable floating around the operating system that contains the users password.
Also note, you have to give the full '/sbin/' address of mount, the only commands you can use in login scripts without giving their full path name are the ones in the '/bin' folder.
Okay, so save this file as something descriptive (like mapfolder.sh) and save it somewhere universal (I like /Library/Scripts/). When you're saving it, make sure it's in plain text format (so if you're using TextEdit to create it you'll have to go 'Format->Make Plain Text'), change the owner to 'root' and the make sure root has full access (in particular execute privileges):
(in command line type)
sudo chown root <Name Of File>
sudo chmod 700 <Name Of File>
Now we'll bind the script to the log in:
(in command line type)
sudo defaults write com.apple.loginwindow LoginHook /path/to/script/<Name Of Script>
And now, as long as you've done everything correctly, you should be golden. Restart and try logging in as a user (any user at this point, if you want to make the script more advanced and check if the user is local or not there are many good ways that can be found in other peoples scripts (in particular analyzing the Network Home Redirector script is fun and rewarding) but that is beyond the scope of what I'm trying to demonstrate here) and see if everything works. If not keep tweaking the script until its happy.
Finally I want to address some of the issues raised in the other posts:
The Applescript option is overkill in my experience (when OS X mounts a remote volume it does all kinds of background error checking and what not that i've found slow things down significantly) and the login items in the users Preferences are very hard to maintain with a large user base. If you do get something with applescript working you can always use osascript to port it to the command line and send it out via ssh, radmin, or any of a number of programs designed to help this kind of mass management. Finaly, the Login/Logout hooks are computer specific not user specific so you will have to deploy to every computer on the network and once again, without more functionality, it will run for every user who logs in.
Sorry about the state of my spelling, hope this helps everyone.