Atleast on the previous N8x0, the charge goes up to 100% (4200mV and <=100mA current), then it sits a long time, the voltage will be below 4100mV when it initiates charging again, bringing it up to no more than 4100mV, and disconnects again until voltage has dropped. After the first peak of 4200mV, it never goes back there unless you disconnect it from the charger and use it (and just cycling the charger cable doesn't seem to make it charge to full 4.2V either).

Also, on the N8x0, the charger was software controlled and used a slow PWM to regulate the charging. That is, pulsed charging. Without the battery installed to soak up the excess voltage coming from the charger, the device crashes/shuts down almost instantly.. I suspect they haven't changed their setup much, if at all for N900.
A not so fun side effect I noticed is that when the battery has aged alot, and the internal resistance has increased dramatically, the voltage over the battery will rise much higher than you'd feel comfortable with during the "on" cycle of the PWM. I discovered there's a safeguard in it though, if the voltage goes over 4.5-something, the battery management software panics and aborts, dsme notices the battery management entity is no longer running, and reboots the device. Reboot loop ensues until user either replaces the battery with a newer one, or uses a lower rated charger

As far as I can tell, the 2mm charging system made it possible to accept a wide range of voltages and types of chargers with the bare minimum of components and heat generation, while sacrificing battery hotswap capability. The 2mm charging spec has a voltage and current windows extending from 4.2V to about 10V, with 5V+-0.2V marked as not allowed. This is from memory, so the exact figures might be different What the advantage with N900 that only does USB voltages is I'm not sure..