Skeuomorphic design vs modern and/or futuristic design.
My $0.02 is this: As long as making a design skeuomorphic adds to the audience’s usability of the application, software, or website, then it’s fine. Maybe even good. As soon as it limits the usability or creates confusion for the audience then it’s bad.
In short, the same criteria as deciding whether any other technique, design pattern, or tool is appropriate.
My personal opinion is that in the case of iCal etc it has been a bad thing. There’s too much loss of good functionality and it’s a poor model. The fact of the matter is that “real paper calendars” are a pain to use. Why emulate the limitations?
Designers also need to be careful of what they’re emulating and why. The usual argument is that people are familiar with physical versions of what our programatic software does. That’s not always a good assumption. Kids these days aren’t going to know what a vinyl deck or cassette looks like or operated like. They won’t have seen a Filofax or a floppy disk, and to be honest I’ll be surprised if many have used a paper calendar. How many have seen a mixing desk?
I think the use of the ideology at the moment has more to do with a desire for textures and cohesive “realistic” aesthetic than it does about any seeming superiority of experience in modeling a real object. I think, in general, the trend Apple are taking at the moment is a mistake. Not always, but often. If the things you like about the approach are the graphic richness, then add that in some other way that doesn’t compromise functionality and the inherent flexibility of the digital medium.