I already showed the link to the Photoshop GUI.
Here are some other links with Iphone elements:
A4 paper to print for sketching on an drawn Iphone:
Elements in different formats, like calendars
this is a whole collection of links of different Iphone situations:
especially this link is interesting for icon design:
be careful though, because the appstore is asking for an icon with the exact size of 57 x 57 pixels.
wallpaper, that is examples of what to do on this format of 320 x 480:
more abstract wallpaper: http://www.iphoneachtergronden.com/Wallpapers-Ontwerpen/Wallpapers-Geometrische-Vormen/
and why not use an app to make a sketch: is this (free) one good: Tanzaku Pad free, makes pdf’s from scribbles….