I was talking with someone about iPhone and Android apps and they said there was something around called "Titanium" that could convert an iPhone app to an Android app and vice versa; being that they are written in different languages (amongst other nuances) I found this a little hard to believe.
I'm assuming he meant this
From what I can understand, you can't "convert" an existing app from one to the other, but if you build an iPhone app based off of this API THEN it can be converted to the other?
Am I right in my thinking?
You're almost right. In Titanium you build your app using JavaScript (and HTML/CSS for webviews - sort of webpages) and it converts it to native code for both platforms - Obj C for iOS and Java for Android. However you still need to write a significant amount of platform specific code because not everything will work right out of the box on both platforms. And yes - you can not convert existing apps using Titanium to anything.