I got increase from 10-15H to 30H just executing two commands in root shell:

echo "1" >/sys/power/sr_vdd2_autocomp
echo "1" >/sys/power/sr_vdd1_autocomp

Interesting, it looks like sequence has matter - I tried it in past but started from last week I occasionally set /sys/power/sr_vdd2_autocomp first, before /sys/power/sr_vdd1_autocomp and it produces this dramatic improvement. The setting sr_vdd1_autocomp first improves only around 30%.

I test it around 1.5 week and it created me now some (pleasant) problem - in the morning (after 24H) the battery still keep around 1/2charge now and I need to bring a charger to work or recharge it anyway. I hesitate to recharge it in 1/2 capacity ... just knowing the battery lasts longer if it is recharged after it comes red status...

Disclaimer: this trick may cause your N900 unexpected crash and reboot - it is not debugged well yet by Nokia. But try it and if it doesn't - you can be pleased.