Using GPIO as a PPS input

Hey All,
I'm interested in using my AtomicPi as a GPS-disciplined time server. I have a few GPS receivers that output a PPS pulse as well as RS-232 for the NMEA data. On a traditional PC with hardware COM ports, the RS232 and PPS can be directly connected, with the PPS line going to the DCD pin.

For the AtomicPi, which doesn't have traditional COM ports, I can just use an RS232-USB adapter for the NMEA strings (or an RS232->UART leveler like a MAX2323 going to the UART on the Pi) - either way the timing for the NMEA data is not very important. The PPS timing is critical, though, and I'd need to run it directly to a GPIO pin.

Has anyone done this, or anything similar? I suspect I'll need to compile a new kernel, which is fine, I just don't want to have to reinvent the wheel if anyone else had started on this.

Thanks!
John