I've read through the entire thread on the 400d/XTi, and have seen this question asked several times. So far I haven't seen a straightforward answer. You can of course remove the stuff from the CF card, but the boot flag changes that resulted from running testfir.fir appear to be permanent. And it also appears that these changes do affect at least one Canon utility program that's used to transfer pictures from the camera to the computer.
So it appears at this point there's no way to return the camera to its original pre-testfir state, even by reflashing the entire Canon v1.1.1 firmware.
But if would be nice if someone who, unlike me, really knows how the 400D hack works could either confirm my understanding or set me straight.
Also, even if the above is true, I wonder if it might be possible to create "untestfir.fir" which really would undo everything that testfir.fir had done.