No Conditional GET for Google Reader, StumbleUpon

Conditional HTTP GETs save bandwidth and computing time by allowing RSS readers to only download feeds when they've changed. Most bookmarking/sharing services support this feature (, Ma.gnolia, FeedBurner, NewsGator, etc) but I was surprised to find the neither Google Reader nor StumbleUpon do. This means that every time that LinkRiver checks a Google Reader shared items or StumbleUpon feed for updates, it has to download the entire feed and look for new stuff. Argh :-(