This is probably true for companies targeting casual users. Facebook and YouTube are great examples.
But I can't see people switching to mobile for the bulk of their serious, work-oriented computing unless the hardware experience drastically changes. I would never program on an iPhone or iPad if a computer were available. Nor would I want to do serious video editing, or CAD. I probably wouldn't even want to type a book on an iPad unless I had an external keyboard and mouse--and at that point, how different is it from a laptop?
Granted, most of these use cases can't be done on the web today. But that may (or may not) change. And even now, there are professional tasks that I'd much prefer to do on a laptop or desktop than on a mobile device--for example, updating Basecamp to-dos.
I also wonder whether this mobile vs web distinction will persist in the long run. Imagine a future where the most popular way to access Facebook is by clicking an icon on your iPad, which launches a Facebook mobile version in Safari or a very thin native app built around UIWebView. Is this mobile, or is this the web? At that point, I think the distinction becomes meaningless.