Which keyboard is best on mobile phones?

It seems that mobile phones are the main place where UI experiments are still being done. One of the types of UI experiment that recently drew my attention was in keyboard design.

Palm Treo

I just got a Palm Treo from work. That Treo has a built in QWERTY keyboard with horribly small keys. The keyboard is always available, together with the screen. And although that sounds convenient, it does mean that both the screen and the keyboard are small and the device is clunky.

GPhone

Compare this to the design of the first Google phone: the T-Mobile G1. When using this device as a phone the keyboard is stowed away under the screen. When you want to access one of its many PDA functions, you fold away the screen and the mini QWERTY keys become available. So while in phone mode the device is a lot smaller, but in PDA mode it is bigger.

iPhone

Lastly there's the iPhone solution: a virtual on-screen keyboard. There on-screen keyboards were pioneered by Windows PDAs about a decade ago, but leave it to Apple to breathe stylish new life into an existing concept. The screen on your iPhone is big and can be used for normal operation. Until you need a keyboard, then the screen is split into two: the top section remains a normal screen and the bottom section becomes a virtual keyboard. The virtual keys are as miniaturized as their physical counterpart. But they do miss the tactile feedback you get from the other devices. So while it looks endlessly better, inputting text on the iPhone is probably slower than on the others.

So what do you think? Which is the better way of inputting text on your mobile device? The Treo approach of an "always on" keyboard? The way the Google Phone hides the keyboard during normal operation? Or do you prefer the iPhone's virtual keyboard?