When using the browser, you don't have the sort of instant gratification that you can get by using the Search charm to immediately pull up what you want to see.
That being said, it does work in the browser. However, for some content, you can only view the HD versions in an app, and not in the browser.
At one point, Microsoft did show an icon for Instant Video on their start screen for a demo they did (which I can't find a picture of at the moment, sorry), but they didn't actually open the app. So my guess is that it's coming, but who knows when.
Edit: Actually, wait, now that I'm trying it again, it's telling me that Flash needs to be updated for Instant Video streaming to work at all.