Ok, having now tried noflushd it seems to do a pretty good job of getting the disk on my Sony Vaio C1XE to spin down, and it will stay spun down while doing compile cycles on a small test program (actually, the whole source to cdparanoia, so not that small): while true ; do make clean all <packaging commands> ; done doesn't make the disk spin up. :-)
Mobile-update won't work on kernels 2.2.11 and above, as the sys_bdflush call will make it do_exit(0) on first call. And noflushd _requires_ 2.2.11 or above, so noflushd and mobile-update will be mutually exclusive. Also, I didn't set the noatime option for mount and this all still seemed fine. Further, to address the spindown of disks when on AC power/battery, noflushd will let you have multiple timeouts and send it SIGHUPs, so you could use apmcd to detect power changes and send it the required signal to keep things as you want for AC/battery setup. </success report> Dave