RxD TxD equivalent pins in USB
USB cannot be used as simply as a serial port. There are many layers of software protocol that must be supported.

Some microcontrollers have some USB support built-in. Even so, I suspect that using USB for serial communications is not as simple as you might think.

See also
USB / Serial converter without FTDI Chip
http://www.fourwalledcubicle.com/LUFA.php