According to Apple, you should be able to run iOS 6 on an Phone 3GS, an iPhone 4, an iPhone 4S, a fourth-generation iPod Touch, an iPad 2 or a third-generation iPad. Yes, this means that folks with older iPod Touch devices, the original iPhone, or the iPhone 3G are officially out of luck.
Now, that's the official word. But it's worth noting that — like some other updates — this particular iOS version might leave some of the older supported device feeling a bit sluggish. Keep that in mind, especially if you're using an iPhone 3GS, which was already left feeling a bit slow after the jump to iOS 5.