In recent months, several of the big browsers have either released <track> element support with WebVTT or are providing experimental builds with support.
Microsoft provided <track> element support in the Internet Explorer 10 Platform Preview 4 to developers on 30th November 2011. IE 10 supports basic versions of both the WebVTT and the TTML format.
Google Chrome 18 added <track> element support behind a flag in about November 2011. You can activate it by going to chrome://flags and “Enable element”. Google Chrome only supports WebVTT.
There is extensive documentation at HTML5Rocks.
Apple released <track> element support in Safari 6 in July 2012. Safari also only supports WebVTT. Like Google Chrome, it builds on the implementation in WebKit, which is trying to be fairly feature-complete for WebVTT.
What about Firefox?
In any case: there is still lots to do. Several of the browser implementations don’t support the full feature set yet. Where they do, they may not be interoperable since the spec may have been interpreted slightly differently, or may have changed since the implementation.
We’re starting to create a test suite with example WebVTT files that test different features. The simple “show text at certain time interval” support is certainly interoperable, but some of the more advanced layout features may not be.