Hi, I am currently developing an App using a 3GS as a testing device, however do not have access to a iPhone 4. I want my App to be optimized on the iPhone 4 so is there any neeccesary chnages I would need to include to make sure my App looks beautiful on the Retina display? I test my App in the Simulator (iPhone 4) and my images (backgrounds etc...) look quite pixelated. I dont know much about the Retina display at all, if someone could kindly advise me on a good approach I'd be very grateful. Many thanks.

The retina display is the same physical dimenstions as the 3GS at 320 x 480. However, the resolution is double at 640 x 960. This means you can create 640 x 960 images and use them as backgrounds for crisper results.

Would you reccomend creating 640 x 960 background images instead then? Or might it be too much to download or load in-App? If I just doubled my resolution on all my images, e.g. Buttons, Headers etc... would that compliment the Retina well? Im just weighing up changes as if the quality of the image is far greater i'll make the effort to replace the images in my app.

I generally don't bother with this. There are some screens that can benefit but generally, normal size iPhone artwork is fine. You said you're experiencing pixelating...this isn't good. Downloading the larger images isn't that big a deal if you save them for 'web' But if you try to download massive files it can get sluggish. The first time the background image loads it'll be slower, after that it'll be fine. Kind of a tradeoff. How many backgrounds are you using? Most mobile apps should't be using more than a few (why fill up the device?) If you're only using a few, add them to the project intead of pulling from a URL unless you need to update them over time. Artwork that can be added to the proejct does improve performance.