I have written an IFilter dll that returns text from my application's file format. I registered it on my local system, and Windows Search correctly returns results with it. I registered it on my SharePoint 2007 server, rebooted, and it doesn't seem to find anything inside the file. Documentation says that all I should have to do is to register it in the same way IFilters are registered for other Index Server implementations (such as Windows Desktop Search)... and that SharePoint will pick up on that as a secondary source of search filters. When that didn't work, I tried setting it up in the first source that SharePoint uses for IFilters (modeled it after the instructions in http://support.microsoft.com/kb/925765). Rebooted again, and set SharePoint to crawl, and it still didn't produce search results.
I have my file type defined for crawl as is spelled-out in http://technet.microsoft.com/en-us/library/cc261843.aspx.
I've looked through the event logs, and through the detailed logs in C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\LOGS (which has sharepoint search messages)...
EDIT: I have it working. There are 2 flavors of SharePoint: WSS and MOSS. I was told that I was working with a MOSS installation, and the registry entries were present for that, so I was modifying there. I modified the WSS registry area instead, and it worked.
I still need to find out why, contrary to the documentation, just registering the DLL and aiming its HKCR. to it via its PersistentHandler for IFilter linkages didn't work.
I also need to better understand the relationship between MOSS and WSS with respect to search. I suppose, when this gets instaled by a SharePoint Admin, I can just install to both registry sections if they're there (search entries for WSS and search entries for WSS).