Wouldn't js click handlers work just as well?

You follow a canonical link to the resource, get a real page back, with real links, but js click handlers to enable AJAX-goosed speed for those with javascript enabled? And given that they imply a fallback for those times when javascript fails, aren't they actually better?

And GMail is a bit different than Twitter. It handles inward-facing data; content that no-one particularly wants crawled and wouldn't benefit much from caching.