Don't want to disappoint you, but most web based systems don't offer native client. They rely on iPhone almost full Safari version as web browser and that's it.
DabbleDB officially confirmed the system didn't have native app. Therefore this affects the possibility of working within the system through Safari web browser in their knowledge base.
Zoho Creator announced they had optimized web layout for iPhone and even created special mobile version.
I've also found indirect confirmation on TrackVia forum the system works on iPhone.
I guess Caspio, TeamDesk and WorkXpress are also available through web browser on iPhone and iPod touch.
Check out the good news now.
Recently Intuit guys have announced on their blog their partner MCF Technology Solutions presented new iPhone application in called PocketBase. Instead of accessing QuickBase via the iPhone’s web browser, MCF Tech has leveraged QuickBase’s API to present QuickBase applications with native iPhone functionality. Currently the application doesn't allow to add/remove records, you can view it only.
I think this is the step in the right direction, because working with such an app is far better than through browser. Anyway mobile devices have a way smaller screen and require other approaches to resolve this issue.
What do you think?
Update from Andrew Catton:
Dabble DB pages are optimized to work both with iphone and android (we think this is the right way to do it, as the pages interfaces are much better suited to a small mobile screen than the core app, and you're more likely to be quickly entering data or checking a pre-defined view in this scenario). More details here.