Mats posted an update on his Sniff-A-Lot for Bus Pirate:
So, I found my Bus Pirate and then made a RS-232 breakout cable with a male/female DB9 and tapped off rx/tx from pin 2/3 and gnd from pin 5. And hooked it up to the first serial device I had in reach – a magstrip card reader/writer (cloning credit cards much? ^_^)
The reader/writer was working even after I plugged in the tap cable and I was able to see both the data sent from the computer and the replies from the unit on the Bus Pirate terminal window. Success!
So the with this unit the Bus Pirate is able to read data from a real high-voltage RS232 serial port, and this on both the tx and the rx lines.