One of the most effective ways to reduce the coupling between PCB traces (to reduce crosstalk) is to put the reference plane close to the traces.
I have always used a ground plane as the reference plane.
Could a power plane be the reference plane and would it still be effective?
Yes, but not as effective as a ground plane. There is capacitance between a conductor (trace) and the reference plane, with a power plane there is added inductance from the power supply. The other problem is slots, as your more likely to have slots in a power plane as a reference plane, slots are bad because they redirect the return current and create more inductance which slows the signals down\higher rise times.
Changing Reference Planes.
When a signal trace changes from one layer to another on a PCB, the return current path is interrupted since the return current must also change reference planes (see right hand figure below). The question then becomes how does the return current get from one plane to another? As was the case for the split planes mentioned above the interplane capacitance is not usually large enough to be effective, so the return current will have to flow through the nearest decoupling capacitor in order to change planes. This obviously increases the loop area and is undesirable for all the reasons previously stated. One solution to this problem is to avoid switching reference planes for critical signals (such as clocks), if at all possible. If you must switch the return path from a power plane to a ground plane you should place an additional decoupling capacitor adjacent to the signal via in order to provide a high-frequency current path between the two planes for the signal return current. This is not an ideal solution, however, since the return current must now flow through a via, a trace, a mounting pad, a capacitor, a mounting pad, a trace, and finally a via to the other plane. This adds considerable additional inductance in the return path (typically 5 to 10 nH).
Source: http://www.hottconsultants.com/techtips/pcb-stack-up-6.html
Yes it could.
The impedance between power and ground plane should be very low, thus making the power plane a virtual ground plane from an AC perspective.
You will need very low inductance bypass capacitors, immediately adjacent to where the high-speed trace changes from over_VDD to over-GROUND.
simulate this circuit – Schematic created using CircuitLab
