There's a big one against the back wall in the Tomorrowland Arcade (aka the gift shop that Space Mountain dumps you into). As previously mentioned, it's also not too hard to find an outlet here or there, and no one seems to mind if you use them (I just sat on the floor sometimes next to one). I used the outlets in the baby stations a lot, too.
That said, one of the best purchases we made for Disney trips was our external battery charger. We have two iphones (iphone4 and a 4S) and we were able to charge up the external battery and charge on the go all day. When one of us got low, we just plugged it into the battery in our bag. The one we bought easily charged our phones entirely at least 6-8 times so we only really had to recharge the external every other day or less. It was BRILLIANT. Definitely a good way to spend $40 if you like to use your phone a lot at the parks (we take a lot of photos with them). We bought the Limeade Blast L130X off of
Amazon (I'd link it, but it won't let me.) We got the midsize one - 13000mAh - and it was more than enough for us.