Since the Samba servers 3.0.24 through 3.0.25a which exhibited this problem may have had problems with parsing a "DFS path" (in this case for a path based call to set the file server), could someone check if the write problems go away if the mount is done with the "nodfs" mount option (would require fairly recent cifs client, perhaps even 2.6.27 kernel, to use that mount option) - that changes how the client sends paths from "\\server\share\dir\file" to "\dir\file" and may work around the problem if the server bug was caused by the processing of dfs paths
Since the Samba servers 3.0.24 through 3.0.25a which exhibited this problem may have had problems with parsing a "DFS path" (in this case for a path based call to set the file server), could someone check if the write problems go away if the mount is done with the "nodfs" mount option (would require fairly recent cifs client, perhaps even 2.6.27 kernel, to use that mount option) - that changes how the client sends paths from "\\server\ share\dir\ file" to "\dir\file" and may work around the problem if the server bug was caused by the processing of dfs paths