In these cases, you must use the null value: alt="" (that was at the very end of what I wrote previously). If there is no attribute, the assumption is that the author has neglected to provide one, and generally the name of the file is read (I cringe when I think of all the times "shim dot gif" was read to someone on my old sites!). The idea here is to indicate that "yes, I've thought about it, and no, there's no alt information to be included here."
I believe there are many times where you wouldn't want the alt attribute to be used precisely because there ARE disabled users. Is it necessary for them to hear a description of every graphic that they can't see? If the graphics are just there for the look of the site, I would think it would be better for the text reader to just skip it all together.
There was some debate as to whether it should be alt equals space, or alt equals null, as some screen reader technologies--and more important to a lot of web designers, Bobby--ignored the null. The consensus on this was that a space could actually be content, and additionally, that null made the most sense long term and most screen readers recognized the null value. I use null, but in the end, space is probably okay, just so long as you have something. All image tags are required to have an alt attribute.