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If you have made iTunes not ask you about buying iOS Apps in the “Are you sure you want to buy and download …?” dialog-box (see below), you could get that warning back by using the “Reset all warnings for buying and downloading” feature on your iTunes Account information page.
Quote from the iTunes Store – Terms and Conditions:

1-Click is a convenient feature that allows you to make a purchase from the Services with a single click of your mouse or other input device. When accessing the Services on your computer, 1-Click purchasing may be activated via the dialog that appears when you click a Buy button. (You may reset this selection at any time by clicking Reset Warnings in your Account information). When accessing the Services on your Apple-branded products running iOS such as an iPad, iPod touch, or iPhone (“iOS Device”), 1-Click is activated for each transaction by tapping the button showing the price of the product, which reveals the Buy button. When 1-Click is activated, clicking or tapping the Buy button starts the download immediately and completes your transaction without any further steps.

Step-by-step guide:

0. Go to the iTunes Store in iTunes, and sign in (if you aren’t signed in already).
   
1. Click on your Apple ID (your e-mail address) in the upper right hand corner of iTunes.
   
2. Enter your password in the password field, and then click the [View Account] button.
   
   (Version of picture without labels).
3. Click the [Reset] button in the “Reset all warnings for buying and downloading” part of your iTunes Account information page.
   
   (Version of picture without labels).
4. You should now see this dialog box the next time you try to purchase something in iTunes.
   

(No, I didn’t read the iTunes Store T&C when I started using iTunes Store. I just googled for “Are you sure you want to buy and download” and got to the iTunes Store T&C, and then found out how to reset this warning).
(Tags (for search engines): One click purchases, One-click purchases, 1-Click purchases, 1 Click purchases, Apple, iPhone, App Store).

The hack connected to a 12 VDC USB charger

I discovered an easy hack to force an iPhone 3G[S] to charge on a regular USB-charger with just one resistor.
To force charging you have to force high the levels of the data pins 2 (D-) and 3 (D+) (white and green wires) by connecting a resistor of about 20 kΩ (or 68 kΩ as I used) between the data pins on the USB cable. This resistor would act as a pull-up resistor. You don’t need to connect this resistor to VCC (+5 VDC) since the iPhone outputs +2.84 VDC on the data pins when it is connected to a power source. This is enough to force the levels of the data pins high. To do this hack, I recommend using an USB extension cord, instead of ruining an iPhone USB-cable, since you of course wouldn’t be able to use this as an regular USB-cable afterwards.

Schematics

Remember to leave the data wires going to the male USB-connector end (the one you plug in to your power source) unconnected, you might seal the ends with insulating tape or glue. This way you could use this cable to be able to charge an iPhone on a computer that is in standby-mode or a computer that supports the sleep-and-charge USB feature, which would let you charge your devices even when the computer is turned off.
This hack would probably work on other iPhone models too, the iPod Touch, Classic, Shuffle & Nano models, and the iPad. It might even work on other devices needing a signal on the data pins to be able to charge.

More information

Links to interesting articles about iPhone charging, where I found out how easy it was to force an iPhone into charging mode:

Charging my iPhone with my hacked USB cable

Apple iPod, iPad and iPhone dock Connector Pinout – AllPinouts has some information about force charging using two 10k resistors connecting D+ and D- to VCC
USB – Universal Serial Bus Connector Pinout – AllPinouts also mentions that one could use 10k resistor to force charge USB devices.
Modify a generic USB car charger to charge a 3rd gen iPod Nano – Instructables Instructables article about modifying a car charger to be able to charge an iPod Nano. Also use two resistors.
How to Charge an iPhone – tzywen.com Mentions how to force charge an iPhone, and shows a proof-of-concept charging circuit schematics.
DIY iPhone Charger – tzywen.com A DIY iPhone charger using 4 resistors, made from the proof-of-concept shown above.
DIY iPhone 3G Charger – tzywen.com another DIY iPhone charger using 4 resistors. Also mentions why an iPhone needs a pull-up-resistor to be able to charge.

Disclaimer

I take absolutely no responsibility for what might happen to your iPhone, your charger or even yourself using the method mentioned above. You do this solely at your own risk.

DIY – Force an iPhone to charge on a regular USB-charger by Håvar I. Henriksen is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.