IIRC, after the installation, you have to edit your /boot/grub/menu.lst and add vga=normal on your kernel line on both parts of the kernel list:
kernel /vmlinuz-2.6.8-3-686 root=/dev/hda2 ro vga=normal
I usually do this at the boot prompt at the beginning of the installation:
boot: debian-installer/framebuffer=false
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