One method of using hard copy blue print drawings in a digital environment is done by using a high resolution photographic image of the drawing. The photo is then manipulated to meet the end use dimensional requirements using photo editing software tools like Photoshop CS4.  This tool is used by engineers for 2D and 3D measurement and allows the user to extract real quantitative information from digital photographic images or drawings.  The tool can assign a known measurement to any part of an image or model, then scale proportionally to match other known measurements on the drawing. This means you can import a photo of an object, enter a known dimension from the photo, such as the length of an edge, then create measurements of other features of the object, such as height, width, depth, distances between parts, etc. This process sets a measurement scale for the image in inches/feet or millimeters/meters (only one scale can be used in a document at a time).

Taking this approach one step further is to edit the image removing the dimensions and text call outs, then insert the image as a background in a popular CAD tool.  The CAD dimensions can be added manually and scaled off the image.  Another option is to trace the image outline suing CAD generated geometric features like lines and arcs.

The image is a raster pixel format and the CAD drawing is vector (mathematical) format.  There are also service vendors who will convert the images to vector format for a fee.  A number of useful services are listed below.  Be sure to evaluate these vendor options carefully and do not send your only hard copy of a drawing to a vendor.

Able Software R2V -
http://www.ablesw.com/r2v/index.html
Vector conversion for scanned maps with tools optimized for mapping and geographic information systems.

Arbor Image Draftsman Cutting Shop -
http://www.arborimage.com/cutgshop.htm
Raster to vector software for stand-alone operation on Windows platforms.

AutoTrace -
http://autotrace.sourceforge.net/
Platform independent program for converting bitmap to vector graphics distributed under GPL.

Consistent Software WiseImage -
http://www.csoft.com/index.cfm?menuid=103
Line of hybrid graphics editing and raster to vector conversion software, as stand-alone Windows programs.

Easy Trace Pro -
http://www.easytrace.com/
Cartographic raster to vector conversion software with topology creation, checking, and editing tools.

Elgorithms MagicTracer -
http://www.magictracer.com
Hybrid graphics and raster to vector converter for Windows that has image editing, vector cleanup, and conversion tools.

GTXRaster R2V -
http://www.gtx.com/products%5Cdetail.asp?id=36
Raster to vector add-on for AutoCAD and RasterDesign compatible with AutoDesk's Image Support Module.

Raster to Vector Software -
http://www.raster-vector.com
Converts scanned drawings, maps, and raster images into vector files for editing in CAD applications.

RasterVect Software -
http://www.rastervect.com/
The raster to vector conversion program that turns scanned raster images into vector format on Windows systems.

Siame Editions Vector Eye -
http://www.siame.com/
Raster to vector converter that exports to SVG (Scalable Vector Graphics), PostScript and EPS.

SoftCover Scan2CAD -
http://www.softcover.com/
Scan2CAD raster to vector conversion software with OCR text recognition and editing tools.

Softelec GmbH -
http://www.softelec.com/
Providers of the HybridCAD software for raster to vector conversion and AutoCAD line tracing.

SoftSoft.net WinTopo -
http://www.softsoft.net/wintopo/
A raster to vector conversion program supporting topographic and GIS tools for Windows and Linux platforms.

Trix Systems, Inc. -
http://www.trixsystems.com/raster-to-vector-conversion-software.html
Converts scans of engineering drawings and logos to DWG, DXF, HPGL, or IGES.

VextraSoft Vextractor -
http://www.vextrasoft.com/vextractor.htm
Windows raster to vector conversion tool for CAD and GIS systems.