I probably shouldn't have said trickle charging. That's the wrong term. As you said, bfksc, as a Li-ion battery is charged, the closer it gets to full, the lower the current the charging circuitry delivers.
It's done this way because Li-ion batteries have a very stable discharge voltage. Older rechargeable batteries (like a NiMH AA battery for example) would output 1.5V only briefly when completely charged. By the time it's half full, it would be down to outputting about 1.3V. This made it very easy to measure its charge level based on voltage. Li-ion batteries hold a very steady 4.2V per cell generally from "100%" down to as low as 40%, which means output voltage is a very poor way to measure its charge state.
To prevent overcharging, the charging circuitry significantly lowers the charging current when the batteries output is at 4.2V per cell. The moment the battery is full, the voltage will drop slightly as the ions saturate around the anode (this is actually bad for the battery but is unavoidable). Once this saturation point is reached, the charging circuitry cuts off the power. Instead of bumping up against that saturation point over and over again, the Nexus S (as well as several other modern devices) lets the charge level float just below it.
bfksc, you say overcharging a Li-ion battery is impossible because it has special charging circuitry integrated into it. While it's true that it has its own integrated charging circuitry, it is still possible to overcharge it. This circuitry protects against several things, including overcharging, but it cuts out at 4.3V per cell, well over the natural maximum output of lithium cells of 4.2V. It has to allow this higher cutoff otherwise the battery would never charge past 40-70%. Continuing to charge past the 4.2V cutoff causes ions to saturate around the anode and eventually begin to plate around it (known as lithium plating). Once this plating occurs, it is irreversible and lowers total capacity due to the loss of free lithium ions.