Here are few modification that allow a very quiet boot, nothing will appear on the screen before the login prompt.
Thus if you start a visual application before that (in /etc/init.d for example) you will not see anything on the screen before your application starts.

First modify the /boot/cmdline.txt like this :
dwc_otg.lpm_enable=0 console=ttyAMA0,115200 kgdboc=ttyAMA0,115200 console=tty3 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=noop rootwait loglevel=3 logo.nologo vt.global_cursor_default=0

here are the details of the changes :
console=tty3 redirect all the post messages to the third console (hit CTRL + ALT + 3 to see them after boot).
loglevel=3 make it less verbose, only errors are reported
logo.nologo disable the RaspberryPi logo on boot
vt.global_cursor_default=0 Disable the blinking cursor.

Moreover you can add disable_splash=1 to /boot/config.txt in order to disable the rainbow splash on power on.

At the en, vous can completly disable the prompt #1 by editing the file /etc/inittab and commenting the following line :
1:2345:respawn:/sbin/getty --noclear 38400 tty1

That’s all !