Does this constitute prior art? It looks like someone filed an application and walked away from it. What happened here?
Taking the parts of the question separately:
No, this is an application that never resulted in a patent.
Prior to what? The determination of something being or not being prior art is relative to some specific patent or patent application in question. Is is veritably a published document and would be useable as prior art against anything filed after its publication date. (Some simplifying)
It must have been fairly early in its field judging from the large number of later patents that cited it. ,
Actually the inventor filed this, received multiple rejections over a two year period and responded to each rejection. The one response I looked at was very far off the required format. The responses were also deemed insufficient to overcome the rejections. The last rejection was a so-called "final rejection" that needed a particular type of response (and more fees). The required response and fees were not submitted. The application went abandoned in June of 2006.
He may have really had something valuable and new when this was filed ten years ago. If he had professional patent prosecution help he might have received a patent that would be valuable now.
The way to find out for yourself what happened is to use the USPTO's Public PAIR database. A quick search on Ask Patents will find some very good answers that include step-by-step instructions.
What happened is this application was written and pursued by the inventor himself without the help of a professional. The last thing he submitted was classified as a "miscellaneous incoming letter" in which he said the examiner never understood the invention and asked the patent office to tell him what to do to keep the application alive.
The next occurrence recorded is the patent office properly declaring it abandoned. I have not looked into the substance of the case, just its progression.