I have an idea for a webapp for the iPhone but its unknown to me how much data can be stored in mobile Safari’s SQLite db. I tried searching through the Apple docs but found nothing:
Safari Client-Side Storage and Offline Applications Programming Guide: Using the JavaScript Database
It’s as the other posters say. You’re only limited by the drive space on the device.
You also need to consider your in memory footprint though. There is a finite amount of memory on the iphone, and in general it’s quiet small, so the amount of data/hydrated objects you’ll be able to have in memory is another potential limitation for your app.