Building for [Mobile or Web] or [Web and Mobile]

Just started to look into HTML5 and how it plays into our plans for app development and what a jungle out there. Take for instance and read some of the comments on http://www.avc.com/a_vc/2010/11/html5-mobile-apps.html and be careful not to be sidetracked like I did when you read about all those great apps people build for both web and mobile.
What do you do as an app developer. Do you start building for Web or Mobile first or do you get ambitious and target both at the same time with something like Responsive Web Design: http://www.smashingmagazine.com/2011/01/12/guidelines-for-responsive-web-design/
Can you at all be sure that you have the same audience on both Platforms or should it really should it be different functionality?
For now we are using two different development environments (and teams) and we are targeting slightly different audiences with our mobile/pad app than with the full blown web app, but I'm still chasing the good example of how to do one app for multiple platforms and audiences.