According to the Yslow developers, even thse replies could be made far mor cache friendly by using the correct headers, especially by removing the ETags-header that forces a conditional GET on every acccess and replacing it with an Expires-header in the far future that allows to reuse the cached version without server interaction (assuming the images are not meant to be highly dynamic, then ETags would make sense).
So from a caching perspective, HN has got it exactly backwards: it doesn't use ETags on the dynamic pages, where it might help, but uses it on the static content, where it might be detrimental. But I wouldn't bet on the gains you'll get from fixing this to be measurable.
Well, HTTP doesn't really have versions other than as a shorthand for header defaults. For example, Connection: close handles implementations that assume HTTP 1.1, iirc.
An Expires header would help a lot for arc-generated pages served in response to requests that aren't associated with a logged in user.
A question that must be asked before investing any time in adding to an existing application: would it solve an existing problem? The Right Thing isn't always necessary and often not worth the time.
ryan$ curl -I http://ycombinator.com/images/y18.gif
HTTP/1.1 200 OK
Date: Tue, 24 Mar 2009 05:04:58 GMT
Server: Apache/2.2.10
Last-Modified: Sun, 18 Mar 2007 17:03:07 GMT
ETag: "7fa02b-64-42bf6725588c0"
Accept-Ranges: bytes
Content-Length: 100
Content-Type: image/gif