This is starting to become a trend, but it needs a strong driver. The Windows 8 App Store may provide the push needed to get HTML5 adoption up for applications. It's a different ball game in the web, but I think we'll start to see the web playing a bigger role in the marketplace.
Just curious: Why the down vote?
It's sad when company acting as someone for open and standard web is doing this. And Google is doing this user-agent sniffing across all their products.
Maybe Google is just playing safe and only launches when they are 100% sure the browser supports the app.
Also, some features have been blocked for a long time for no apparent reason, for example background image on search. It worked from the day one (when you changed the user-agent), there is nothing scientific about it, yet it is still blocked years later.