Possible Duplicate:
Can I safely charge my iPhone 4 overnight on a regular basis?
Is leaving the phone on the charger overnight really really bad for your iPhone?
Possible Duplicate:
Can I safely charge my iPhone 4 overnight on a regular basis?
Is leaving the phone on the charger overnight really really bad for your iPhone?
Nah. Electricity is PULLED to devices that use it, not PUSHED from the plug. The phone is smart enough to stop pulling when its battery is full.
EDIT: However, an adapter can provide incorrect voltage for a device. Don't take what I said there and wire your iphone power adapter to a 220 source, for instance. You can OVERDRIVE a device with too many volts, but you can't send it "too much" of the right voltage, because the amperage is a function of the device's draw. I know I just said that assuming that you understand the difference between volts and amps, but there it is; they're different.
No, leaving it charging overnight will not have any adverse effects. All you need to do with the iPhone's lithium battery is be sure to drain it once a month for longevity.
Read Apple's iPhone Battery page.
[Similar answer for Apple's lithium laptop batteries, iPad batteries, etc.]
Also see Apple's Lithium-Ion Batteries page for more info.
The answer is it's just like an laptop: If it's left plugged in all the time, then the battery WILL eventually die. So the answer is yes, it's OK to leave it plugged in, but not all the time.