It's relatively easy to setup samba shares on XBMC Live, but it does require bashing out some commands on the shell. First, you'll need to install samba using apt-get:

sudo apt-get update
sudo apt-get install samba

Then, you'll want to configure your shares by editing the the file named smb.conf, placed in the /etc/samba/ directory. For example, I added the user folder and media folders to be available shares by running the following command:

sudo nano /etc/samba/smb.conf

Update: It looks like this has changed in the more recent versions of Ubuntu that the XBMC Live distribution is packaged with (10.04). You can restart the samba service using the old method:

sudo service smbd restart
sudo service nmbd restart

Scroll down all the way to the bottom of the file, then paste in the following (use right click in Putty to paste something from your clipboard):

[xbmc]
 path = /home/xbmc
 public = yes
 guest only = yes
 writable = yes
 browseable = yes
 force user = xbmc
 inherit permissions = yes

[media]
 path = /media
 public = yes
 guest only = yes
 writable = yes
 browseable = yes
 force user = xbmc
 inherit permissions = yes

After that, restart the samba server using:

sudo /etc/init.d/samba restart

Note: I updated the samba configuration above to include the 'force user' and 'inherit permissions' features as I was having trouble with newly created files not getting the right permissions and logging into samba.

Also, when accessing the shares from Windows, if you left the default security options you can use the account 'guest' with no password to access the shares. Otherwise, you can play with the settings to force a local linux account to be used in order to access the shares.