I've had a look at the datasheet for the pic, and it states
"Flag bit RCIF is a read-only bit, which is cleared by the hardware. It is cleared when the RCREG register has been read and is empty."
So the WHILE isn't actually resetting it, reading RCREG is.
I also found the datasheet for the PL-2303, it rx's at TTL (5v levels) and tx's at 3.3v - so that's some concrete information, but doesn't really help with what seems like inversion...
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