Hy,
I had the same problem, which I could solve by editing the files
/etc/cups/mime.convs AND
/etc/cups/mime.types
After uncommenting the lines
application/octet-stream application/vnd.cups-raw 0 -
in mime.convs
and
application/octet-stream
in mime.types
I could print via SAMBA.
I think the same could be achieved by adding '-oraw' to your print command in smb.conf
(print command = lpr -r -P'%p' -oraw %s
or whatever

)
Hope this helped
regards
Tirsales