Nevermind, it doesn't work with grub2.. 

Adding dns-nameservers to /etc/network/interfaces resolved it, thank you!

Thanks for sharing your story :)

Grml ships the resolvconf package out-of-the-box, so you should be able to set
"dns-nameservers 8.8.8.8 8.8.4.4" in your /etc/network/interfaces file and that
should play well together with the "nonetworking" boot option. Can you please
verify that this solves your issue?

I'm confused why grub2 works in your situation though. Can you please verify
that the same boot options are present with grub2 boot vs. isolinux boot in
/proc/cmdline?

regards,
-mika-

Long story.. ;)
I'm experimenting with grml-live and different boot methods (grub2, isolinux) to create a custom 
rescue image. We don't have dhcp, so a script creates an appropriate /etc/network/interface, but 
that will be overwritten if 'nonetworking' isn't set. According to [1] and with grub2 boot 
method, this works => networking is set up correctly, and dns is set, too. If I choose 
'isolinux' boot method, dns is gone. Don't know what should be the right behaviour

[1] http://git.grml.org/?p=grml-live.git;a=blob_plain;f=templates/GRML/grml-
cheatcodes.txt;hb=HEAD

Hm, you're using the boot option "nonetworking", then the dns boot option indeed
doesn't work. Why do you use the nonetworking boot option? :)

regards,
-mika-

/etc/resolv.conf is empty when dns=* is set

root@grml:~# cat /etc/resolv.conf
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
#     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN

root@grml:~# dmesg|grep "Command line"
[    0.000000] Command line: initrd=/boot/grml/initrd.img boot=live nosound 
nonetworking dns=8.8.8.8,8.8.4.4 noeject noquick nostats services=ssh noconsolefont 
nogpm tz=Europe/Berlin lang=us vga=normal nomodeset live-media-path=/live/grml/ 
bootid=grml001 apm=power-off nomce  BOOT_IMAGE=/boot/grml/vmlinuz

resolvconf -u doesn't help