need screen-refresh (VBlank) event for psychological reaction-time experiments

There's a long tradition of doing computer-based experiments in neuroscience, psychology and other human-behavioural sciences.
Doing such experiments over the web, instead of in the lab, with a mass-audience, is a big opportunity.
It is important to know when exactly a user perceived a stimulus (e.g. an image or sound).
For graphics, it's important to know when they where sent to the monitor, i.e. the VBlank time. Normally the same time as the buffer-swap.
Hope you can create a simple callback or event for this, carrying the timestamp of the last VBlank.

Related: http://lists.w3.org/Archives/Public/public-web-perf/2011Jul/0002.html