I'm looking at my phone. It is a Samsung Galaxy S so it has a pretty display with a lot of icons on it. For example the one to take pictures is a camera, the icon to play music is a disk, looks like a CD, with a musical note in front of it. The icon to show video has a film reel. The icon to get at the ebooks is a hardback book. Messages is an open envelope.
Notice anything? Well in a few years all of those icons will be meaningless. Even now I'm not sure how many people remember film reels we used to use for movies. CDs are fast being replaced by downloads, books (ie dead-tree books) will be replaced by ebooks. The camera is, of course, now something that looks more like a phone. How long before the idea of a camera that isn't inside a phone vanishes. Sure the pros will probably still use specialised devices, but the rest of us will rarely see those.
All of the icons will then be just... a phone.
Though the cultural images do last longer than the things themselves. The near-universal signs for toilets are the silhouetted figures of a man and a woman... well a human wearing a skirt and a human NOT wearing a skirt to be precise. But there are not so many places where women wear skirts of knee length depicted. They wear trousers, shorts, short skirts, long skirts, loose skirts, tight skirts. They almost never look like the silhouette.
So in 20 years or so will the kids look at the movie reel icon and think 'video'?
Maybe they will. It does also have the triangular arrow pointing to the right that video clips have. Maybe they've future-proofed that one.
The Twitter icon and the K9 (email) icons are based on the brand name, not so much illustrating the function as who provided it. That works, and maybe they will morph into more generic icons so that we'll think K9 even when we're using a different email client (assuming K9 becomes that widely popular). But Biro did it with pens, so it isn't out of the question.