From my discussions with people that should be somewhat in the know on Li-Ion batteries:
- do not discharge below 3.2V, do not ever overcharge above 4.2V (well, perhaps 4.25V)
- no memory effect indeed, so no need to wait for it to be discharged entirely
- phones have controller chips on the battery that will avoid any charge taken out when it hits the preprogrammed level (which can be 3.2V, 3.4V, 3.6V, whatever); at that point, it will appear dead - BUT when connecting a charger, some charge will trickle into it, and some (older/old style) phones will therefore take up to 15 minutes to show a sign of life!! New phones have a bypass system on that battery charging mechanism, so they can have a 'dead' battery that's on life support/being revived, yet still immediately boot to avoid the customer returning to the shop with a non-broken device merely suffering from deep-discharge
- some phones have a so called gas-gauge (N9 included) which continuously monitors the current out of the battery (measures the voltage over a small resistor in series with the battery), to allow to do real estimates of the charge level, since determining the remaining energy is not reliably doable anymore due to the voltage being "too" stable over discharge (voltage over time/usage curve is 'flat' for a long time, then suddenly dropping off; due to the flat section, from the voltage it's hard to estimate when the drop will come) - this gas-gauge also allows to fine tune the charge current to extend the battery lifetime; not sure in how far the N9 charging mechanism exploits this possibility

So no matter what the battery "nuts" say, it all depends on what the "pro's" actually implement (and by "pro" I mean those who build the systems / paid workers, not implying that they know more by default).