I'm creating buttons for my app specifically for the iPhone 4S and the iPhone5. For some reason, any objects that are circular in nature appear jagged on the phones. For example, a location marker which I created in Photoshop doesn't look jagged from within Photoshop. However, when exported as a .png and used as a button in iOS, the edges around the curve of the location marker are jagged. Is this a problem with the resolution?
I'll take a guess that you're exporting as PNG-8 with a small number of colors, which chops off any pixels below a certain transparency and leaves the resulting image strongly aliased (the technical term for "jagged"). PNG-8 only has one level of transparency; it's an all-or-nothing proposition, but using a high value for the number of colors can give an acceptable result, depending on the background.
Use PNG-24 to achieve smooth anti-aliasing. This is what is recommended in the Apple iOS Human Interface Guidelines.