ergophobe - 4:26 pm on May 5, 2010 (gmt 0)
This was common in lots of CMS 3-4 years ago. I thought everyone had fixed it. Are you seeing this with the latest Joomla version?
Ironically, I did a t search for "Joomla Valid 404" and got a link to the extension directory and it returned... a 404. A valid one too. Here's my abbreviated HTTPLiveHeaders output:
GET /extensions/490/details HTTP/1.1
HTTP/1.1 404 Not Found
Date: Wed, 05 May 2010 15:43:24 GMT
So I wonder if there's something wrong with your install.
Also, I believe most Joomla users are running shSEF404 (Is that the one?) in addition to core SEF
I don't know if it helps with 404s.