Coin-cells, or a better implementation, IMHO, capacitors (so you never had to mess with changing the backup battery). I'd kill for either of these on my tablets, and have even considered some hacking to add it... But the current situation being what it is, there's just no way Nokia's going to give us that revolutionary old feature. I suspect this shift is partly linked to the switch from off-the-shelf AA/AAA cells common in many older devices to the ubiquitous Li-ion-polymer cells.

All cellphones are designed with the dual assumption that battery replenishment is by charging, never changing, and that state is trivial (so rebooting is no problem). The latter is revealed in the awkward compromises of putting SIM slots, microSD slots, and even data cable contacts behind the battery. Together, these make adding a backup power system doubly pointless, and even though Maemo devices to date seem to have ducked the second, the first seems firmly in place.