Some people in the rumorsphere are claiming that the next generation MacBooks will sport "glass trackpads". The implication is that they'll have little LED displays under your finger, like an iPhone interface, doing... goodness knows what.
The truth is that anyone with an ounce of user interface knowledge will intuitively understand that this idea does not work. When you're using a trackpad, you're looking at the screen, not the trackpad itself. So you simply would not see whatever is on the trackpad display. Besides which, the position of the trackpad means that it's largely obscured by your hand when you're using it. In order to use a trackpad as if it was an iPhone, you'd need to hold you Macbook up to your eyes using one hand and use the trackpad with your other, staring at the trackpad, rather than the screen.
The best illustration of how flawed this concept truly is comes from the recent fake spy shots illustrating the Mac OS X Dock moved from the main screen to the trackpad. If we can get past the obvious problem that there wouldn't be space for many apps on a Dock that size, can you really imaging performing tasks like dragging a document from the main screen down to the trackpad?
It's more likely that, if the new Macbook introduces glass as a material at all, it will be used in some other way: maybe a glass screen like the iMac, or if the trackpad itself is glass, this would probably be just for the feel of the material under the finger, rather than because Apple wanted to put a screen on the trackpad.
Whilst an iPhone-style trackpad is an amusing idea, let's be honest, it isn't going to happen. All that this kind of rumor serves to produce is the frustrated whining that we've come to know and love on the MacRumors messageboards after every major Apple product announcement. So for the record, let's also clear up that the new MacBooks won't solve the current economic crisis, won't offer a cure for cancer, and won't usher in a new age of world peace.