I built an iPhone app called Commit that tackles this very problem. It is in the review process right now and should be released in the next couple of days.
Because the iPhone developer can start with an existing userbase, and the developers of the web versions can now offer an iPhone app.
I run http://weekplan.net which is in a similar space and many people ask me for an iPhone/Android app. I have made a mobile stylesheet but a native app would definitely be a Win/Win/Win (iPhone developer / Users / Me)
I had this idea about 2 years ago and didn't bother building it when I looked at the competition in the space (evidently many people have had this idea). Now I see these guys have 500,000 users... I guess the space was larger than I thought :)
You can read a bit about it here: http://nathanbarry.com/a-little-bit-each-day/