I can't believe that people still fail to grasp the engineering feat that Apple has accomplished here in regards to power management and engineering.
Let's consider the iPhone's size. The iPhone is literally half the size of a Treo (11mm thickness compared to 23mm thickness). Even smaller when compared to the older models.
Now, what would you imagine the overall space would be that the battery occupies in the new iPhone. Considering the entire size of the phone and the components involved:
8GB of RAM
A huge 320 by 480 multi-touch screen
Quad-band (MHz: 850, 900, 1800, 1900) radio
Wi-fi
Bluetooth
Camera
Antenna
Speakerphone
Headphone jack
iPod interconnect
SIM card
Main processor
DSP processor
Multitude of circuit boards
What kind of room can they possibly have left for a phone. Based on the size of the iPhone, a person would have to imagine that half of it would be dedicated solely to the battery to get 5 hours from it.
Yet, this is impossible. Look at the list of stuff they have shoved in there (in addition to the bits of electronics I know nothing of that are required for such a device).
My point is that the battery is quite small and to engineer the ability to allow the user to swap batteries in and out would impact the resulting size to a much greater degree than if for example Palm were to make this same change given the Treos existing size.
Do you get it? The iPhone's battery would lose a greater overall percentage of its mass to engineer an iPhone design that would allow for a swappable battery because the space to hold the battery is so small to begin with. The iPhone would end up with 2-3 hours of battery life.
I am glad Apple chose to engineer a built in (non-swapable) battery.
Besides; the whole idea of making these "smartphones" like a Treo or a Blackberry or the iPhone is to give the advantage of convergence. I will be able to just grab my iPhone and go - not worrying about also unplugging that extra battery pack and carrying that around with me too.