So how come my older computer could read serial data from a PIC operating at 5V? Was it not "RS232"? I had ground between them connected, so the data line wasn't "floating".

Also, hypothetically speaking, couldn't I convert TTL to an "approximate" RS232 with an op-amp using 6+V with a 3V reference as ground? Or even a transistor tied to a 6+V supply with a 3V ref tied to the RS232 ground?