Fairly comprehensive discussion on the Apple Forums. Even a very competent electrical engineer there agree's that if you charge it quicker you are not doing your battery any good:
https://discussions.apple.com/thread/2580579?start=45&tstart=0
Apple supply it with a certain charger for a reason, use it. I can attest to the fact that the faster charging with a higher rated charger (approved by apple) ruins my battery much faster!
If that was true then Apple would not list it as compatible with every single iPhone, including the original:
http://store.apple.com/us/product/MD836LL/A/apple-12w-usb-power-adapter?fnode=3c