Hi,
Since you're not interested in position one simple way would be to count pulses from one channel for a certain amount of time. The direction could then be determined by polling channel B at the rising edge of channel A - or the other way around. What kind of speeds we're talking about. A couple of 100Hz or several 100kHz or even more? What else is the PIC going to do?

Which PIC are you using? If you haven't decided on one yet then one option is to have a look at the 18F2431 series, it's got a quadrature encoder interface peripheral.

/Henrik.