Done as follows:

Parition a 8BG USG-Stick with fat32 /dev/sdX1 and /dev/sdX2
Made /dev/sdX2 bootable
# grml2usb --mbr-menu --verbose grml96_2011.12.iso /dev/sdc2

Boot from stick at a 64bit machine (TP t61p):

grml32 runs as expected
grml64 doesn't boot (How to debug?)

booting from grub-menu the machine doesn't boot at all and tells:

...
Please load kernel first
...

or so.

Installing via
# grml2usb --mbr-menu --verbose grml64_2011.12.iso grml32_2011.12.iso /dev/sdc2

every flavour is bootable.

Elimar

-- 
  Numeric stability is probably not all that
  important when you're guessing;-)