To my way of thinking the continuing gotcha with the iPhone is that the web based applications are exactly that -- you need to be on the web to do anything including looking at your data. Your data is not on the iPhone.
I think if you hack the phone you can install app's that save data to the phone but you are, of course, of in dangerous terrritory having voided your warranty. Apple are also suggesting hacks may break the phone permanently.
So good though wifi coverage is where I live I just don't like the idea of no being able to see my data because I can't connect. What do you do in a plane?
Note this is not true of iCal and the Apple applications on the iPhone neither is it a criticism of the people developing web based apps for the iPhone as they are just working with Apple's rules.
I don't have access to an iPhone (not out in Australia yet --- it takes awhile to get everything turned upside down I guess) so I would be happy to be proved wrong about this
Thanks - Michael