There is a fairly serious problem with the Pololu USB to RS232 adapter, however it is easy to fix. When the converter is plugged into a USB port, the default seems to be to set TX high (regardless of whether a terminal or other program has allocated the port).
If the converter happens to connected to a port pin on an unpowered MCU, it will power the MCU via the input protection diodes and may cause reset problems. Powering up and down the MCU may also cause the USB port to hang, requiring one to kill and restart a terminal program, for example. I have now determined that this happens with either PIC or AVR processors.
The simplest fix is to put a resistor between TX on the USB adapter and RX on the MCU. I used 27K ohms, which seems to work. An isolation diode also works.
Added 4/25/07: The problem is worse than I thought. I just fried the USB RX input when I switched on the PIC to which it was connected (the USB converter was unplugged from the PC port). I now have 27K resistors between each connection. You may need to go higher than 27K – the port still sometimes hangs when the PIC is powered up and down.
These are expensive lessons! Does anyone need a TX-only USB-RS232 converter? Free for postage…