For a cell phone, the wall adapter just supplies a constant voltage, like any other wall wart, and the phone has all the current regulation and charge monitoring circuitry internally.
If the charger is small, light-weight, and cool to the touch during operation, it probably has a switching regulator inside. If it's heavy, bulky, cheap, and warm to the touch during operation, it's probably a linear regulator.
5 V micro USB chargers will be the standard in the future. With any charger, the voltage applied to the phone is fixed, but the phone can adjust the amount of current it draws. Some supplies can provide a lot, but some (like plugging into a laptop's USB) can only supply a little. There are a few different ways to tell the phone how much current it's allowed to draw. The USB-IF battery charging standard has the charger short the D+ and D- pins together. iPhones, on the other hand, require certain voltages applied to the D+ and D- pins. Others use the 5th ID pin to indicate a dedicated charger. Otherwise, without some kind of identification of the charger, a USB-charged phone should only draw 100 mA, in case it's connected to a low-power computer port.
As for "Protection against back current flow", a simple Schottky diode will do. The charger circuitry drives current into the battery by applying a larger voltage than the battery's voltage.
And the phone's internal circuitry takes care of this, as well as switching off power (or applying a float charge?) when the battery is fully charged.