The part that has me really scratching my head is that, while I persistently get this permission denied error when I try to smbmount the partition, I can access it fine with smbclient, not only listing files, but saving them to the local system!
Out of curiosity, have you checked your firewall settings on both machines? Make sure that 137-139 and 445 are open. Most people seem to be forgetting 445 these days. That and the windows firewall likes to cause issues....
-- "They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety'' Benjamin Franklin 1775