Happened to stumble across this thread on the Ubuntu forums, and thought it might help. It explains the steps that happen behind the scenes:

In Windows the client's username and password is automatically sent when it browses for shares - this is done without the user's knowledge. That forces Samba to deal with the sent credentials even though it's a guest share that requires no authentication.

When that username is passed Samba will search through it's password database for that user:

• If there is no match to the username the client user is tagged a "Bad User" and converted ( mapped ) to the guest account which by default is "nobody".

• If it finds a match to the username and there is a samba password that matches the one sent by the Windows client then the Windows user automatically gains access although not as an anonymous user which is why you needed to add "force user = nobody" to your share definition.

• If it finds a match to the username but the samba password does not match exactly the password that's automatically sent by the Windows client then you will be prompted for a password - even for a guest share.

Try adding force user = nobody to your share definition, and see if that does it.

Edit 02/20/2013:

Is testparm returning an exit code of something other than zero? All the same, I would go ahead and give that area of the config a good, hard look. Also, I'm not sure how case-sensitive smb.conf is, but every example I see (for example) of map to guest = Bad User has the B and U capitalized. Check-out the Samba man pages for the options you are using, and double-check everything.

In order to set up guest access in Samba, you need to set up a user that it will pretend to log in as. So, let's say you want to share files on /mnt/somepartition/files publically. Your configuration might look like this:

[public_files]
    comment = Public files
    path = /mnt/somepartition/files
    browsable = yes
    guest ok = yes
    writable = yes
    guest account = someusername
    create mask = 0775
    directory mask = 0755

What this saying is "Create a samba share on \mymachine\public_files that is viewable to anonymous users (not hidden like user files generally are) and enable it for write access. Anonymous users can access this share by using someusername's credentials. When files are created, make them globally executable but restrict write access globally. When directories are created, make them globally executable but only writable by someusername."

Once this is done, you will need to create a smbpassword, per your question. To do this, ensure first that the user exists within your server. If the user doesn't, create it:

sudo adduser someusername

Once the user exists, create a samba login:

sudo smbpasswd -a someusername

A couple of things to keep in mind: the directory that public_files points to will need to be READ accessible to someusername. Make sure you set the permissions correctly. If the directory is owned by you but still want to make them available, add someusername to a common group and then change the group ownership.

OK, I have found an answer myself.

As this is absolutely not obvious from the docs and HOWTOs and whatever, the reason this thing asks for password is because it cannot map guest user to the owner of the directory being shared.

I have NTFS partitions which I need to mount RW so I used the following setup in my /etc/fstab:

/dev/sdb1  /media/disk1  ntfs defaults,noexec,noatime,relatime,utf8,uid=1000,gid=1000 0       2
/dev/sdb2  /media/disk2  ntfs defaults,noexec,noatime,relatime,utf8,uid=1000,gid=1000 0       2

The most important pieces of config are uid and gid (maybe only uid, don't know). They are set to the UID and GID of the user jonnie set up on the server (obviously not root). So, when ntfs-3g will mount these disks, everything will be owned by him.

After that, I have added this user to the Samba registry (or maybe created new identical one, don't care):

# smbpasswd -a jonnie

It asked for password, I have entered the same as for the main system.

After that, I have added the force user and force group settings to the smb.conf:

[global]
  workgroup = WORKGROUP
  netbios name = HOMESERV
  security = user
  map to guest = Bad User

[disk1]
  comment = Disk 1 on 400GB HDD
  path = /media/disk1
  browsable = yes
  guest ok = yes
  read only = no
  create mask = 666
  directory mask = 777
  force user = jonnie
  force group = jonnie

[disk2]
  comment = Disk 2 on 400GB HDD
  path = /media/disk2
  browsable = yes
  guest ok = yes
  read only = no
  create mask = 666
  directory mask = 777
  force user = jonnie
  force group = jonnie

So, most important piece of config relevant to me was force user.

Courtesy of the Samba HOWTO

I recommend to create a dedicated user for that share and specify it in force user(see docs).

Create a user (shareuser for example) and set the owner of everything in the share folder to that user:

adduser --system shareuser
chown -R shareuser /path/to/share

Then add force user and permission mask settings in smb.conf:

[myshare]
path = /path/to/share
writeable = yes
browseable = yes
public = yes
create mask = 0644
directory mask = 0755
force user = shareuser

Note that guest ok is a synonym for public.

remove

valid users = "@domain admins", "@domain users"

from [global], only use it in shares that are not "guest" enabled