Modules that don't build on x86 against 2.6.20(-grml):

acx100 (error: macro "INIT_WORK" passed 3 arguments, but takes just 2)
bcm5700 (error: 'CHECKSUM_HW' undeclared (first use in this function))
cpad (error: macro "INIT_WORK" passed 3 arguments, but takes just 2)
fglrx (error: expected declaration specifiers or '...' before 'mlock')
ivtv (error: macro "INIT_WORK" passed 3 arguments, but takes just 2)
madwifi (error: this is the location of the previous definition)
nozomi (error: macro "INIT_WORK" passed 3 arguments, but takes just 2)
openafs (error: 'struct task_struct' has no member named 'todo')
rt2570 (error: #error Sorry, this nice piece of code was not designed with SMP 
in mind.)
vaoistat (error: expected ')' before string constant)
vacom (makefile error)

Notice: 'touch include/linux/config.h' inside kernel tree helps compiling some 
modules.

What works fine:

KPKG_DEST_DIR=/raid/Grml/kernel/ module-assistant -f -l '2.6.20-grml' -k /raid/
Grml/kernel/linux-2.6.20-grml build drbd8 fglrx-kernel ndiswrapper nvidia-
kernel qc-usb realtime-lsm rt2400 rt2500 rt2x00 sl-modem spca5xx sysprof tidev  
misdn kqemu nvidia-kernel-legacy

Notes:

ipw3945 fails too but a new upstream version works fine (needs manual 
interaction though).

exmap and fglrx need symlink /lib/modules/2.6.20-grml/build pointing to kernel-
tree

misdn*.deb is placed in /usr/src/ instead of KPKG_DEST_DIR