Is it OK to charge my iPhone with my iPad charger? I have an iPhone 4 and an iPad.  Since the iPad really charges best off the wall charger I keep it plugged in, but I usually just charge the iPhone off the computer.  Although frequently if the iPad is charged I will charge the iPhone with the wall charger.

Now the Apple sales rep said he heard from someone who heard from someone that that would burn out the phone because the iPad charger is 10 watt instead of 5 watt.  It is true that it is twice as many watts.  I said that I thought it shouldn't burn out the phone because a device should only pull as many watts as it needs, so the watts were irrelevant.  He said that made sense, but he didn't really know.

So I thought I would ask the experts.  Has anyone had their iPad charger fry an iPhone or iPod?  Am I correct in my understanding of watts in chargers?  Is there an official word about the iPad charger causing trouble with other devices?  Have I already voided my warranty?
Your assumption is correct. Your iPhone has an intelligent charge controller on-board and this controller will only draw as much as it needs. 

Sure, the iPad power supply can supply twice as much power as the iPhone needs, but it should not be dangerous by any means.

I don't know if there is an "official" word on this, but I would be genuinely surprised that it voids your warranty. Apple engineers are very smart and know that users will interchange powersupplies if they recognize that the connectors are physically compatible (as is the case here.)

Long story, short: you should be fine.