A design fad that's been around for awhile now is the "blueprint" look: people drawing architectural blueprints/sketches either as a background to their roughed-out icon, or integrating a blueprint into the icon/design itself. What does the blueprint represent?
- Unfinished, in flux
- Creativity, making something brand new
When should you use the blueprint look in your icon?
What you use to represent metaphors in your icon design is completely up to you, however it's clear that most icons that incorporate a blueprint are indicating that "this application is used to build stuff" whatever that "stuff" may be. For Apple's suite of developer tools it means building other applications or widgets. For your blueprint icon it could represent building anything you'd like, but something that a user can actually build within the application.
Here are a few full-size icons that use the architectural sketching metaphor, oh, and can't forget about MacThemes' website!