The MPlayer Christmas Edition for Wii supports SMB browsing. You can configure the login data of the desired SMB share through the smb.conf on the SD card as follows:

ip=192.168.1.100
share=Public
user=wii
pass=somethingelse
port=445

For it to work, you MUST use a dedicated user in Samba. Guest shares won't work. Also make sure you have security=user set in your Linux smb.conf. For more information see this thread as tehskeen.com.