It wasn’t long ago that mobile phones had weak browsers and 3g was a network that the carriers built but without the “killer app”.
Smartphones with amazing browsers changed all of that. The web is the killer app. The browser on iPhone and Android are incredible and only getting better.
So if you are building a consumer web service these days, I highly encourage you to think of the mobile experience right away. Don’t think of mobile as something you’ll get around to doing one day. It has to be a high priority.
That doesn’t mean you have to build a native Android or iPhone app. It really depends on the service. I can’t imagine an app like Bump working without a native app while Techmeme mobile web site is amazing. It often comes down to whether your app requires access to local iphone resources (camera, address book, etc) or not.
At the very least your application/service should be optimized for mobile browsing experience and has an open API.
That’s the minimum these days.