Why do I need mkinitrd with 2.6?

[Jeff Mitchell <jam6@cec.wustl.edu>]

Hello--

For whatever reason I can't boot up a 2.6 kernel (on Sarge-testing)
without using mkinitrd to generate an initrd file.  No Debian 2.6 HOWTO
I've read has this step, and a friend who has build kernels on
Woody-testing has never had to do it.

If I don't have an initrd and appropriate line in /boot/grub/menu.list,
I get the following:

VFS: Cannot open root device "hda1" or unknown-block(0,0)
Please append correct "root=" boot option
Kernel Panic: VFS: Unable to mount root fs on unknown-block(0,0)

Using mkinitrd (having it scan the directory with the 2.6.4 modules),
copying the output to /boot, and configuring menu.list to use it makes
my kernel work just fine.

Can anyone tell me why this is happening?

Thanks,
Jeff