FTDI basic breakboard + PIC18F452
Do you have the output driver enabled on the PIC UART RX pin? If it is driving a high level out, the voltage will go to 5V when the FTDI drives high, and something in the middle due to contention when the FTDI drives low.

(This may be a silly suggestion, depending on how the PIC UART configuration works, in which case I'm sorry.)