Hardware ConfigurationFor Dell C6100 I set these setting in the BIOS
Remote Access = Enabled
Serial Port Number = COM2
Serial Port Mode = 115200 8,n,1
Flow Control = None
Redirection After BIOS POST = Always
Terminal Type = VT100
VT-UTF8 Combo Key Support = Enabled
Note: "Redirection After Boot = Disabled" is required otherwise I get a 5 minute timeout before booting the kernel. Unfortunately with this set up you get a gap in output while the server attempts to pxeboot. However, you can interact with the BIOS and once Grub starts you will see and be able to interact with the grub and Linux boot processes.
For Dell R510/710 I set these setting in the BIOS
Serial Communication = On with Console Redirection via COM2
Serial Port Address = Serial Device1=COM1,Serial Device2=COM2
External Serial Connector = Serial Device1
Failsafe Baud Rate = 115200
Remote Terminal Type = VT100/VT220
Redirection After Boot = Disabled
Note: With these settings you will be unable to see the progress of the kickstart install on the non default console.
Grub configurationIn grub.conf you should have these two lines (they were there by default in my installs).
serial --unit=1 --speed=115200
terminal --timeout=5 serial console
This allows you access grub via the consoles. The "serial" (ipmi) terminal will be default unless you press a key when asked during the boot process. This is only for grub and not for the rest of the linux boot process
SL6 ConfigurationThe last console specified in the linux kernel boot options is taken to be the default console. However, if the same console is specified twice this can cause issues (e.g. when entering a password the characters are shown on the screen!)
For the initial kickstart pxe boot I append "console=tty1 console=ttyS1,115200" to the linux kernel arguments. Here the serial console over ipmi will be the default during the install process, while the other console should echo the output of the ipmi console.
After install the kernel argument "console=ttyS1,115200" was already added to the kernel boot arguments. I have additionally added "console=tty1" before this, this may be required to enable interaction with the server via a directly connected terminal if needed.
With the ipmi port set as default (last console specified in the kernel arguments) SL6 will automatically start a getty for ttyS1. If it was not the default console we would have to add a upstart config file in /etc/init/. Note SL6 uses upstart, previous SL5 console configurations in /etc/inittab are ignored!
start on stopping rc runlevel 
stop on starting runlevel [S016]
exec /sbin/agetty /dev/ttyS1 115200 vt100