Add the following line to /etc/apt/sources.list
# SAMBA 3 Backport
deb woody samba
Backup your entire /etc/samba directory
cp -R /etc/samba /etc/samba-backup
Pin apt to always use packages from backports instead of the stock debian apt database... Add the following to /etc/apt/preferences (you will most likely have to create the preferences file)
Package: *
Pin: origin
Pin-Priority: 999
Update your apt database
apt-get update
Install the new samba
apt-get install samba
You will get a few prompts from debconf. 1 is regarding the DOS codepage and only affetcs DOS (non-windows) clients and the other asks if you want to convert your samba passwords file to the new format (i answered yes). Apt will stop and restart your samba daemons. You may get a few warnings if you have options in your smb.conf file that are no longer used in the new version, just make note of them and remove those lines from /etc/samba/smb.conf and then restart the daemons (/etc/init.d/samba restart).

Test connectivity and delete the /etc/samba-backup directory if everything is working... I kept a copy of my smb.conf from 2.x SAMBA just in case.