Spare Batteries
avc.com
avc.com
Apple probably figured that this was a feature worth ditching because hardly anybody needs it, maybe also because there are (clunky) workarounds (with all those external batteries). The iPods and later the iPhone have an active accessory ecosystem so Apple can do stuff like that.
(In practice, the iPhone 4's battery actually lasts for 1.5-2 days of heavy usage, so it's seldom an issue, but of course that won't be true a year from now.)
Unfortunately it looks like they discontinued it: http://www.amazon.com/Duracell-Charger-Rechargeable-Batterie...
It's a design tradeoff. Not requiring the user to open the iPhone means that Apple doesn't have to design a latch and they can pack components tighter.
I have a standalone charger that plugs into the wall, and half a dozen spare batteries. Rather than plug my phone in to charge, I swap its battery with the fully charged one in the charger. This means I never worry about discovering my phone is almost dead when I'm about to leave the house; recharging to full is instantaneous, rather than tying my phone (and by association, me) to a single spot for hours. Doing things that drain the battery quickly are no longer a big deal. And if I'm going to be traveling for awhile without the ability to plug in, I can take as many batteries as I'll need.
But a lot of cars nowadays basically do this. There's an engine cover underneath the hood hiding all the engine parts, with only a place to add fluids visible.
I spent about $30 on it and now its only $20 money well spent.