Apple replaced Samba with SMBX, their home-cooked application for Windows File Sharing. By default, it doesn't do much logging. If you want to review logs, you'll have to edit the launchd item. Add the two extra ProgramArgument keys in bold below.
beteblanche:~ noahabrahamson$ sudo vi /System/Library/LaunchDaemons/com.apple.smbd.plist
<key>ProgramArguments</key> <array> <string>/usr/sbin/smbd</string> <string>-debug</string> <string>-stdout</string> </array>
Now you can use syslog -w to review your logs in Terminal in real time.