Hi,
No, the high bits are not always 'at 1'.
When doing a bitwise AND both "inputs" must be true for the "output" to be true. Thus Temp = PortA & %11111100 will do a 'copy' of PortA except the two LSB's will be forced to zero. All the other bits will be left as they are when PortA is read.

Besides, it doesn't really matter what you write to the port/pins which are configured as inputs. If, for example, PortA.5 is an input and you write '1' to it it will still read the actual state that whatever is connected to it drives it to - which isn't neccesarily '1'. If you look at the schematic for the I/O pins you'll see that the output driver for the pin is disabled when TRIS is set (ie. pin is made an input). If it wasn't it would short out whatever drives the pin to either Vdd or Vss.

/Henrik.