Second only to commercial real estate's addiction to the fax machine, is the construction industry's addiction to blueprints...
Enter PlanGrid. From a recent TechCrunch article:

PlanGrid is a cloud-based service that delivers blueprints as PDFs directly on the tablet; then people working in the field can use these instead of paper-based versions. When a modification needs to be made, that can be directly noted on the plan, in the app. That is subsequently updated into a new version. PlanGrid’s technology makes the rendering and scrolling of those blueprints significantly faster, too.

Product features include:

Always have the current set with you, no matter where you are
Page based versioning of drawing revisions
Easily upload project files to your PlanGrid cloud account
Calibrate and takeoff linear measurements
Markup your drawings with the annotation suite
Share your markups instantly with your project team
Filter your drawings by system, revision, or however you’d like
Take progress photos and pin them to your blueprints
Attach 8.5x11 pdfs to each sheet for RFIs and specifications

Download the app: iPad