Thanks - I did that, but to no avail! All filesystem drivers are *not* modules, and I re-built with all the ones I could need selected. I still get the 'cannot open root device' thing though, seemingly even when I boot the old kernel!
You did not include support for your root filesystem. You probably had it as module, but to load modules kernel has to read the root filesystem.
I'd try again and spend a little more time with make menuconfig. Initrd should not be used with custom kernel, it is meant for generic we-support-everything setups.