The "genspeed" script looks for all the common baud speeds in /usr/include/$(uname -m)-linux-gnu/bits/termios.h; if B600 were missing from there, you'd see a different error message, along the lines “stty: invalid argument ‘600’”.
The error “stty: /dev/ttyS0: unable to perform all requested operations” suggests that the kernel is rejecting the tcsetattr call; have you by any chance upgraded the kernel recently? -Martin
