Interesting
Selenium removes the If-Modified-Since header. This gets in your way if you are testing ETags.
Speaking of ETags - they are awesome. See Ryan Daigle's article for code.
Help
Just how slow is Mongrel when serving static images?
The answer is - pretty slow. The reason for the question was that Google's crawler tends to keep a single connection open and fire lots and lots of http requests over a period of several hours. With a standard Nginx/Mongrel setup, this would tie up a mongrel for this entire period.
A proposed solution to this problem is to use HAProxy between Nginx and Mongrel.