Seems like a longshot, but... We've got two iBooks in the household (G3 700MHz & G4 1GHz). Lately they both fail to give the low battery warning and just power out when the battery runs out... Drives me nuts when I've got some unsaved files open
This just happened to both machines with iPod Shuffles connected (we've got two of them too) this evening. Could it be that the PMU fails to take into account the powerdraw of the iPod?