When developing for mobile, its important that you are able to support all "in the wild" versions of operating systems and hardware you are targeting. Here at tapadoo, all iPhone projects are tested on iPod Touch, iPhone 3G, iPhone 3GS and iPhone 4. We're also updating our style guidelines for developers, and henceforth all supplied graphics will be required in standard and double-size to support the fancy new screens of iPhone 4. Older hardware is slower, so testing on these is essential to ensure that our clients' apps still give the best user experience possible. We also now have to consider iPad, and the new possibilities it raises with extra screen real estate.
Phew. Luckily, we're stocked up well to support all this; we took a serious delivery this morning. Check out the testing pool. 
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Thanks for reading the Tapadoo blog. We've been building iOS and Android Apps since 2009. If your business needs an App, or you want advice on anything mobile, please get in touch