Very odd problem that I ran across and solved today. Here's the scenario:
MOSS search result pages were fine for full-blown administrator accouts (like mine), but for normal (non-elevated that is, many arn't actually "normal") the item icons would only display broken image links. Odd thing though is that if you pulled the SRC element from my page (the one with the working image), and navigated directly to it logged in as the normal user, it came up fine. Hence the problem was not with the icons themselves, but the page was actually rendering differently.
So after chasing down many paths I ended up in the 12\template\xml directory examining the docicon.xml file. Everything looked cool, but eventually after comparing it to other files in the same directory I noticed that it's NTFS security permissions were different. Rather than try and fix those, I just renamed the old file, created a new file in it's place and then copy & pasted the text (in notepad) into the new file. After a quick IISRESET all was well again.
Now in analyzing how that actually got jacked up, I compared a clean version of the file (from a dev server) with the one that wasn't working. The only difference was the addition of the PDF icon mapping. This was done correctly, but whoever did it (pretty sure I know who, but no need to mention names), must have done some strange or weird MOVING of the file (which in windows, if you are on the same volume retains the source file's permissions instead of inheriting the new parent's). Anyway, the lesson that should be learned here is that unless you really want to move a file (say because it's huge) on the same volume, just be in the habit of doing copies... they are safer.