I found it very annoying to have to go through the whole "this iPhone is synced with another computer..." business every time I plugged in my iPhone to charge at my desktop (it normally is paired with my laptop), or my computer at work.

But I discovered -- inadvertently -- how to prevent this from happening: just use set a passcode in your iPhone settings screen. Then, when you plug it in, iTunes will give you a single dialogue letting you know it can't connect until you enter the code. Dismiss the dialogue, and you've got a charge-only setup.