Performing a remote install on an HP Proliant via the iLO text interface
Debian CDs will by default pop-up a graphical splash screen. Since this puts the system into a graphical mode, you will be unable to see the bootscreen via the iLO Serial-console interface (telnet/ssh). You can however get in a command line mode when pressing the ESC key when the screen displays the ISOLINUX loading message just after POST. (If you were to late to pressing ESC however, you can still interact with the bootloader, so you can blindly configure the system to boot into a text install mode).
The correct command-line to run to get the default iLO serial console working is:
install vga=normal fb=false console=tty0 console=ttyS1,9600n8
This starts the default text-based installer, using regular VGA mode, disabling FrameBuffer and redirecting the console to ttyS1, using 9600 baud and 1n8 stop/partiy/bits serial mode, which is iLO2's default.