Digital vs acoustic is always a great debate especially with beginners like Sal's daughter in the previous post. I have a fairly crummy acoustic and recently bought a Yamaha P80.
Just of couple of points to throw out:

1) The law of diminishing returns. Basically if you spend more and more, your return becomes less and less. For example, say a $4000 piano might be 20% better than a $3000 piano. But a $5000 piano might be only 10% better than the $4000 one. My point is that if I had 5 grand to spend on a piano. I would buy a Yamaha P80 and a $4000 acoustic. Best of both worlds.

2. Few of us are going to win the Van Cliburn competition. For most of us the idea is to play for the enjoyment of ourselves (and perhaps others)...and to improve. No argument that the true instrument is the acoustic, but the digital is a fantastic learning tool. Most agree that some of the most important keys to learning are the use of a metronome, slow play while learning, and hearing ones self. The built in metronome and the ability to record to an on board or midi sequencer really helps. Yesterday I recorded the opening bars to the Moonlight Sonata 3rd mvmt slowly, then speed up the sequence. It sounded fantastic. My daughter heard the playback from another room and exclaimed Dad, is that you!?. Well...yes and no. The point is that I received tremendous positive feedback. Over time, as my velocity increases while maintaining accuracy, I should be able to sound pretty good.

3. Other sounds. Its great to play some of the Bach pieces that I learned on the piano with the pipe organ or harpsichord. I also give the ear some relief during practice to use other sounds.

4. More practice time at night with the headphones and less driving the family nuts with repeated practice of difficult measures.
It is also great to work on a piece "privately" only subjecting others to a more finished product right from the start.

5. Although the sound of a digital is not the same as an acoustic, one thing I did notice is that it does sound like a great recording of an acoustic. So if I can enjoy the electronically reproduces sound of a CD piano performance, I should also enjoy the sound of my own "performance" (talent notwithstanding).

6. With midi or onboard sequencing one can more easily use sequencer for ensemble play or practice and play a duet with yourself!

7. Finally, if there is more than one pianist in the family having both acoustic and digital alleviates competition.

I actually have three: Accoustic piano, electronic keyboard and mini-electronic keyboard. My normal instrument is the accoustic, but when I want to hear the "Well-Tempered Clavier" in a harpsichord sound I switch to the electronic. I also switch when it is late at night and I don't want to wake up the family.

The mini-electronic keyboard is mostly for my infant sons to play on. But, you know, it's easy to carry under my arm when i'm on the road. Plus, it was cheap. I could throw it away tomorrow and still have gotten my money's worth.