I cannot understand how Bing Community site is implemented. Clicking one of the "All Recent" "Unanswered" "Unread" or "Your Discussions" issues regulare request that refreshes whole page (proxy display regular request) also it changes URL (full URL not just #hash). But page does not flash!
How they achieved this?
For me, it doesn't flash on FF3, but it does flash on IE6. So, I think it has more to do with the browser's rendering.
BTW, a quick test (which works most of the time) to see if a page is fully being reloaded is to select some text (I selected the top navigation text on bing.com) -- if the page reloads, your selection will be lost.
Not sure if this is how the URL gets changed, but you can easily request the new page and change the page contents using AJAX. I'm assuming that's how.
They're definitely doing a full-page refresh. Firebug's Net panel and Firefox's Live HTTP Headers plugin both show a new page being requested as does monitoring the request via a proxy like Charles.
However, according to Y!Slow, the entire operation takes less than a second. They're sending back about 9k of HTML gzipped, and everything else (their sprites, scripts, etc.) is pulled out of the cache. I think you're seeing how fast sending back a really light, gzipped page with cached assets renders in the browser. Bing's done a great job of optimizing this aspect of their site for very fast page loads.