Thank you, I wouldn't have figured out on my own...

<http://httpd.apache.org/docs/new_features_1_3.html>
says:

"Starting with 1.3.15, the server will satisfy directory 
requests with the cache controls ETag and LastModified, if 
IndexOptions includes the TrackModified directive. The 
server will not need to generate the listing if the client 
determines the request has not changed, improving 
performance. Due to its experimental nature, this feature 
is not enabled by default."

(I've got Apache 1.3.12... so far - newest is 1.3.26)