Are power outlets in a home out of phase with each other?
Sort of. It depends on your frame of reference.

If you're looking at the ungrounded (hot) conductor from each receptacle, you'll end up with a 240 volt circuit.  Since it's a single circuit, it can't be out of phase with itself. If you hooked up an oscilloscope to the ungrounded (hot) conductor of each receptacle, you'd get a single 240 volt sine wave.

If you're looking at the two separate circuits (e.g. the ungrounded (hot) and grounded (neutral) conductor from each receptacle), then you'll end up with two 120 volt circuits 180&deg; phase shifted from each other. If you hooked up the oscilloscope to the ungrounded (hot) conductor and grounded (neutral) conductors from each receptacle (4 leads instead of 2), you'd see two 120 volt sine waves 180&deg; phase shifted from each other.

The two sine waves should look something like this.



Because the waves are phase shifted 180°, the electrical potential between the legs (at the peak) will be 240 volts. While the potential between either line and "neutral", will be 120 volts.

If the waves were not phase shifted, they'd be at the same potential (or have 0 volts between them).



So while this is not a multi phase system, it's also not a simple single phase system.  Technically it's known as a "Single split phase system".

All of this; of course, assumes that the ungrounded (hot) conductors are from different legs of the service.