P.S. if anyone replies saying that it doesn't matter because disk space
is cheap, I shall be forced to nominate you as Linaro sysadmin for this
cycle. 
Ha! :) Only for this cycle?
 
I don't quite understand your maths there; if you look at the Ubuntu ARM images
they are 540MB for netbook and 200MB for headless (compressed).
So at 6 boards to support that's ~4.2GB/month or ~50GB/year which is a lot
less scary.

The Ubuntu Images have an extra bit that happens on first boot where it expands itself to consume your entire SD card. 

-Paul Larson