I think it's possible you're confusing the concept of serial (as in "communication", http://en.wikipedia.org/wiki/Serial_communication
) with serial (as in RS232 serial on older PCs). USB data communication is serial.
Depending on the Atmel chip you're looking at, there are many ways to get them to talk to other devices serially. I'll presume you're talking ATmega like in the Arduino, so you'll have SPI and the serial UART on pins 0&1 as well as the ability to implement serial comms on most of the other pins via the SoftwareSerial library. So you'll have many methods of communicating short distances via serial (ie bit-at-a-time) comms.
However in the text here you've mentioned communication with a PC and the Arduino. Do you need a standalone chip that will be talking to both? And over what distances?
Cheers ! Geoff