There are many apps available in the app store which can assist you in learning how to play a guitar but we need to somehow accommodate our iPhone with the instrument. To accomplish that, you should buy the new Guitar Sidekick from Castiv. It has a adjustable mechanism which can accommodate to the device size which means it can also adjust a Sony PSP.
Guitar Sidekick will help you to see notes that you want to play and other references when you are playing a guitar. It is also perfect for play alongs as there are enough instrumental videos on YouTube for that extra bit of guidance from your idols.
Guitar Sidekick is available for only $30 and I think it is a worthy investment. We have no word on its iPad version yet but it would be great to have a thing like that for iPad too. Although your guitar would get heavy with an iPad on it.