I want to create an iPhone/iPod game. I want it to be 3d, but I want to use sprites instead of OpenGL to do this (I assume this would be easier since I don't know OpenGL).
I was thinking of simply layering sprites over top of each other and changing their size to give an illusion of 3d. It doesn't need to be too convincing since the game will be somewhat cartoony.
How can I use sprites as an alternative to OpenGL on the iPhone platform?
You can use CoreAnimation for this. Either using UIImageViews (or plain UIViews) or CALayers.
It's usually the best choice for some types of 2d games (board games, for example), since animation, rotation and scaling are really easy. Just keep in mind that if performance is your concern, OpenGL will always be better.
In addition to Marco's answer I want to add: Not using OpenGL may also tax the device battery a little more. As I understand it, OpenGL ES can be more efficient on a device power supply (if implemented properly). Of course, this depends on how much animation is going to be used with UIImageView, UIView or CALayers, etc.
I'm sure there is a tipping point.
Depending on how much 3d, I'd recommend taking a look at cocos2d. It supports multiple layers, sprites, animations, etc, but is pretty straightforward to pick up & learn. (Much easier than OpenGL to me) The example code is very comprehensive.
I have built a game using core animation with upto about 17 - 20 objects floating about the screen scaling and rotating and performance was fine on the iPhone (make sure you check regularly on the iPhone as the simulator doesnt simulate iPhone memory or CPU speed).
CoreAnimation is pretty simple and really powerful. Use PNG's for images and I don't think you will have to many issues. The real killer of this will be alpha's in your images, this is hard work for the iPhone. So the less transparency you have the better you app will go.