I ran into bug 401652 while trying to preseed a bigmem kernel using the Debian etch 4.0r6 installer.

d-i base-installer/kernel/which-kernel select linux-image-2.6-686-bigmem

This was fixed (post etch) in base-installer 1.83. As a workaround, I added the kernel package to pkgsel/include.

d-i pkgsel/include string ilinux-image-2.6-686-bigmem