I bought my first digital camera (a Kodak LS443 with 4 MP, 3:2 image aspect ratio) in 2003 specifically to take pictures of documents in archives. When the lens failed on that first camera in 2007, I upgraded (Canon PowerShot A640 with 10 MP, 4:3 image aspect ratio). In my experience, a digital point-and-shoot camera is adequate for records office work. With the exception of low lighting conditions, where the much higher purchase price presumably buys more aggressive noise reduction at ISOs of 800 and higher, I would expect a digital SLR to be overspecified for the job.
In regard to resolution, I have found that a 10-MP camera has enough resolution to produce usable photos of 11" x 17" construction plan sheets, provided that the entirety of the plan sheet is in or close to the plane of focus of the camera. When the resolution is that high, it matters less what the image aspect ratio is. With the older digital camera I was working with a much lower pixel count, so I deliberately passed over 4:3 cameras and went for a 3:2 camera because I expected to be working primarily with British office foolscap in the UK National Archives (office foolscap has not been used in Britain in close to thirty years, but is similar to American 8 1/2" x 14" legal size). I understand that the National Archives has now invested in digital scanners and established a service whereby you can scan your own document, the Archives keeps a copy of the scan, and you get access to your scan via email or Web. For US state archives and the National Archives and Records Administration, I think you still have to bring your own equipment. NARA has historically allowed not just digital cameras, but also portable scanners. The Caltrans library (whose photo collections Jake explored) does not allow scanners and charges $10 per copy of an 8" x 10" print done by an outside contractor, but as Jake found, they don't object to digital camera copies taken without flash.
It is useful to have a digital camera which is lightweight and relatively easy to maneuver--aside from the other considerations mentioned, such as lighting and resolution, you need to be able to get as much of a flat original as possible within the plane of focus. A high frame rate is helpful for taking pictures of multiple documents in rapid succession, but it still pays to use the LCD screen to check focus for each shot (as a general rule of thumb, the more of the original that is in the plane of focus, the higher the count of focusing rectangles in the LCD screen). For taking pictures of photos, the LCD screen is also essential for checking that there is no specular glare.
11" x 17"/A3 is pretty much the size limit for a single-frame picture without acrobatics. 34" x 22"/A1 is doable but not without holding the camera above shoulder level or standing on chairs. When the original is larger than 8 1/2" x 11"/A4, it is prudent to take at least one image of the entire surface of the original, and then move in closer for detail shots. The standard program setting will work for entire-surface shots of large originals, but for smaller areas (including entire surfaces of ordinary-sized originals), the macro function will be necessary. Recently I took a complete camera copy of the first construction plan for signing on the Kansas Turnpike (which was shown to me as a set of 11" x 17" mirror-image whiteprints), and came away with 358 shots, of which 41 were whole-sheet shots of the original (which had 38 sheets for signing for the signing contract proper plus multiple sheets of typical sections which were presumably included on a "for information only" basis), with the remainder being detail shots.
As a general rule, the macro function will be necessary for ordinary-sized pieces of paper and anything smaller (including, say, 4" x 6" photos). While there will be some variation from camera to camera, with the macro function on my PowerShot I have found that moving to a normal angle of view (corresponding to 50 mm focal length for a 35-mm camera) will get rid of pincushion or barrel distortion, but won't allow an object of interest to subtend a larger solid angle (and thus take up more pixels in the frame) compared to what can be achieved by moving in closer and using mild wide-angle (corresponding to 35 mm focal length for a 35-mm camera).
For avoiding specular glare on glossy originals, it is helpful to identify the point sources of light in the environment and to try to move the original so that it is equidistant between the two nearest point sources. This is essentially how copy stands are made for professional duplicating work--two light sources are positioned on either side of the original and set to shine on it obliquely so that it is evenly illuminated but reflections off the surface are not visible through the camera lens.
For standard office strip lighting, I have found that ISO 400 is usually adequate. I will go up to ISO 800 for exceptionally poor lighting situations, but I generally prefer not to do it because image graininess is much greater at that ISO with my PowerShot. Archives which rely heavily on natural lighting (through picture windows etc.) can present special problems--you might be able to go as low as ISO 100 during the day and then have to jack up to ISO 800 when the sun sets (as it will during office hours in winter, at a sufficiently high latitude) or cumulonimbus clouds roll in.
It also pays to mind your auto white balance, and set the camera for tungsten if you are working with a blue tabletop or, e.g., one of your light sources is ordinary incandescent while the other is a compact fluorescent. A consistent yellow cast won't ruin pictures of bitonal or grayscale originals but can get annoying.
