Just a note for anyone who encounters is_dir() returning false on CIFS mount points or directories within those mount points on 2.6.31 and newer kernels: Apparently in new kernels they've started using the CIFS serverino option by default.  With Windows shares this causes huge inode numbers and which apparently can cause is_dir() to return false.  Adding the noserverino option to the CIFS mount will prevent this.  This may only occur on 32 systems but I don't have a 64 bit install to test against.