Do smartphones and tablets generally use dynamic or condenser electret mics?
The electret microphone is used but is rapidly being replaced by a MEMS type which has a much flatter profile and can be surface mounted to the pcb. 



A MEMS microphone is a condenser type microphone that comprises a MEMS die and a complementary metal-oxide-semiconductor (CMOS) die combined in an acoustic housing. (The gap is about 4um giving a capacitance in the region of about 0.5pF)). The CMOS often includes both a preamplifier as well as an analog-to-digital (AD) converter.  

The electret is small, light, cheap but with good sensitivity and frequency range. Essentially it is the first cousin to the condenser microphone. Both operate by forming a 'condenser' or plate capacitor. The sound (pressure) changes the plate gap altering the capacitance value (as it does in the MEMS). These changes in capacitance alter the voltage across the plates which follow the sound pressure changes. 

In the electret microphone one of the plates is made from a polarised (charged) insulator (electret). This means that no external high voltage (a few hundred voltage) is needed to charge the plate. Both types are very high impedance and require some form of buffering/impedance changing circuit. In the case of the electret this is built into the microphone using a JFET. In the case of the MEMS the circuit is integrated into the CMOS structure of the microphone. 

The dynamic microphones generate their own electrical signal. Generally these are electro-magnetic types with low impedance outputs (a few hundred ohms).