I've just come across this bug report whilst googling to set up a feisty beta box with a serial console. Things seem to be working. I don't know if this is because Feisty has fixed the problem, however there is one thing that seems out of place in your grub line above.
You have: console=ttyS0,115200n8 console=tty1
I think it should be: console=tty1 console=ttyS0,115200n8
As the doco says it's the last console entry that becomes the input
I've just come across this bug report whilst googling to set up a feisty beta box with a serial console. Things seem to be working. I don't know if this is because Feisty has fixed the problem, however there is one thing that seems out of place in your grub line above.
You have: console= ttyS0,115200n8 console=tty1
I think it should be: console=tty1 console= ttyS0,115200n8
As the doco says it's the last console entry that becomes the input