You're welcome mate. Sad that it did not work though. I think what I use my Xbox the most today is streaming video and playing emulators, ghehhe. XBMC rocks!
I found your problem very strange though. Just a quick test. Let's say you have a file called "my_movie.avi" in your Samba shared folder. Change the permissions and see if that helps:
chmod 777 /home/shared/my_movie.avi
The above path is just an example. Permission 777 is full read, write and execute. Also, make sure that the owner of the file and folder is same you use to connect to samba(I don't think this is necessary, but if everything else fails... we never know ^_^). Let's suppose I have an user called "alex". alex is the owner of /home/share/ and everything on it. It's also alex the name and password I use to login to my samba share from my Windows machine or Xbox. Works fine for me.
If that does not work for you, then I'm afraid that my cards to play here are over =(. I wish you luck though