On: No, I'm not going to download your bullshit app

[fpgeek]

Yes, compass heading can be queried through the browser: http://www.html5rocks.com/en/tutorials/device/orientation/

As of April 2011, the article claims various degrees of support from mobile Chrome, Safari, Opera and Firefox.

There's also plenty of work on application caching and offline support (though, to be fair, that's a very hard problem): http://www.html5rocks.com/en/features/offline