Charge iPhone 3GS with iPad charger?

I just realized the last two times I had hubby plug in my iPhone 3GS to be charged, he plugged it into the iPad charger. The battery charged both times. Is it OK to do that, or will it eventually kill my battery/phone?

No it should be fine.

Its fine, the iPhone will only draw as much current as it needs

Nope, it won't harm it at all. BUt the only thing that won't work is if you get the iPhone5 or ipod 5 charger and try to charge it.