It is important to use the HTTP Last-Modified/If-Modified-Since conventions to help Web crawlers avoid requesting and downloading resources that haven't changed since their last visit – it saves time and CPU load for the DSpace site too. The JSPUI implements this protocol on /handle and /bitstream servlets, since the Item splash page and Bitstream downloads are responsible for most of the load.
The XMLUI needs to do this too:
1. Requests that return the content of an Item or Bitstream must return a "Last-modified" header with the Item's last-modified date
2. When receiving a request with an "If-Modified-Since" header, first compare that date to the Item's last-modified; if the header is later, return a 304 status instead of processing the request.
Note that the XMLUI's Sitemap implementation already does the right thing.
I'll look into implmenting this for 1.6 if nobody else jumps on it first, but right now I only have time to post it.