Yes, more or less three possibilities:
Real mode, open bus: reads last data asserted (the previous value is stored on bus capacitance -- it's like having one byte of DRAM made in a PCB). On many CPUs, the last data fetched will simply be the instruction currently executing.
Real mode, pulled bus: 0xff or 0x0 (or maybe something mixed, but that would be weird?). If the pulls are weak, it could be a mix of the above, depending on timing and maybe noise.
Protected mode: likely the address isn't mapped and a protection flag is thrown. The kernel could just as well map it of course, in which case the above applies.
Open bus is relevant to a number of systems; in fact it appears in a number of classic console games, when abusing glitches to cause out-of-bounds reads. There is such a glitch in Super Mario World, for example (SNES, 65C816 CPU).
Tim