Let's see...

I unplugged the phone at about 8AM as I always do and went to work. At around 9AM I changed profile to silent and remembered to change it back at around 4:30PM only to find my phone dead. So I put a bit of juice back in, booted and had a look at battery eye. It showed on the graph the battery depleted at around 2PM. So...from 8AM to 2PM the battery went from 100% to dead, which is 6 hours.

Normally, I start charging again evenings. The night before the depletion problem I started charging at about 8PM and at that point the battery was still at 40%, which means it only took about 60% of battery juice in the 12 hours it was off charger. Still a lot, but acceptable for daily use. Simple calculation says that one day the N900 takes about (60/12) 5% battery juice per hour and the next it suddenly takes (100/6) 16.7%...without changes between days. No updates, no new software, no different usage profile...after recharging and a boot it's back to normal.

Obviously I wasn't able to check the process list as the device was shut off due to lack of battery juice All I know is what I've done before and after which was...nothing.