<SMALL>I was always under the impression that pianos were "temper" tuned by professionals with an "ear" so that various chordal combinations would all sound "okay" (whatever that is) to our ear. By this I assume that we're talking some sort of compromise between JI & ET ... but this discussions has seemed to indicate that pianos are ET ... if this is so then why do we have piano tuning professionals?</SMALL>
Some people use an electronic device or a set of forks to set what is called the "temperament octave" in the center of the piano. If you were to take the tuning test to enter the Piano Technician's Guild however, neither of those methods would be accepted.
You would have to learn to tune the entire piano beginning with a single fork. To make a long story short, the temperament octave is tuned so that the beat rates in major third and major sixth intervals increase in speed by about one half beat per second, as one proceeds upwards by half-steps.
Since each piano and set of strings is different, accomplishing this even progression of beat rates in thirds and sixths will not generally match pure, exact equal temperament. However, even the Conn Strobotuner manual for piano tuning (not its basic owner's manual) suggests varying from pure equal temperament when necessary, to achieve this even progression of thirds and sixths; it's more pleasing to the ear.
The intention is still to have the piano equally usable in all twelve keys. So, it isn't really accurate to say that piano tuning involves a compromise between ET and JI, at least in that the general intention still involves 12 equally spaced semi-tones.
However, in another way, aural piano tuning does involve at least one basic principle used when tuning by ear to JI: The final authority and method involves beat rates created by coinciding harmonics between different intervals, or the absence thereof -- not a visual readout of pure ET on an electronic tuner, since the two will not always agree.
Notes are tuned progressively more sharp as one proceeds up from the temperament octave, and flat going down. This is necessary because of "inharmonicity," which refers to the phenomena whereby a string's thickness causes it's harmonics (overtones) to be sharp from what they should theoretically be. When you play different note groups together on a piano, the relationships between overtones and the beat rates created by these relationships are more important than whether the fundamental frequencies of all the notes match theoretical equal temperament.
Some authorities also have mentioned a tendency of the human ear to hear higher frequencies as being flatter than they are, so that may also be a factor in why octave stretching in the treble sounds good, as well as the common desire to have more brilliance in that section.
An acoustic piano that is tuned straight to equal temperament with an electronic device sounds horrible. If that weren't the case, you can be assured that professional piano tuning would have been exposed as a colossal fraud long ago, since it's fairly simple --putting aside the physical techniques involved-- to buy an electronic device and tune every note straight to equal temperament.
Many modern piano tuners use electronic devices as aids, but their end goal still follows the general guideline I've laid out.