I seem to recall running into the impedance of an AVR or PIC I/O pin impedance figure a ways back; might have been for the ADC though. In the range of 40-60kohms.
Certainly not for a PIC, like I say there's no set impedance, but the A2D pins have a maximum recommended source impedance of about 2K on most PIC's. This isn't due to the load impedance of the input, but due to the capacitor charging time in the sample and hold.