As I now understand it, this has something to do with framebuffer support. But instead of disabling that in the kernel (which you might want to do if you ever get around to rolling your own, for some really good reason (I'm not a fan -- see why in Section 5!)), you can just pass a couple options to your boot loader.
Debian uses LILO by default, so at this point, you can just use the
following (I don't know how to do it in grub, but I'm sure it's
This means that instead of hitting `ENTER' as usual and watching the
stock kernel load, you will need to type:
(The kernel image, followed by the boot parameters.)
You can now boot up okay, but you'll need to be wary of this when you're glibly `ENTER'ing through menus later in the install -- when you're asked if you want to append anything to the kernel image for the boot loader, you need to type that line (video=vga16:off) again. If you don't, the screen will get split again.
You can fix a split screen if you already have an installed system by editing your /etc/lilo.conf. In your linux stanza, just put append="video=vga16:off" (this is what would have happened if you'd specified it in the base system configuration). Then run lilo -- your changes won't take effect until you do. (This will also tell you if you've screwed it up somehow -- it's good to double-check.)