Please elaborate what exactly you want to know. There's serial console (UART3) via low-voltage-RS232 in N900 (under battery) which is used by NOLO and kernel as console and which might change to a proper RS232 adaper in Neo900 (just about to check that detail in schematics). There's a spec for serial over USB via RX on D+ and TX on D- (or other way round) which probably won't get supported. There's USB ttyACM which is supported even by ROMBL in some form. There's JTAG on SoC which may or may not be available on Neo900. There's even EmbeddedTraceMacrocell / ETB with JTAG and its special bus, to do in circuit CPU tracing.
What do you want to do?

/j