I may be building a mobile app for a client soon. Their primary focus is the iPhone, but my boss would like to be able to target multiple platforms if it's feasible. The app will probably be a large but technically simple business application backed by a web service. So, here's the question as I see it:

What is currently the strongest cross-platform mobile development tool that supports iOS? Would you choose it over native development tools? If you choose native, contrast it with a cross-platform tool you've used.

In addition,

• For a project of the type we're expecting, what's the level of effort for your chosen tool versus other tools?
• What's the actual level of support of the tool for other platforms and their unique look and feel, capabilities, etc.?
• How thorough is the documentation of the product?
• How well do you like the development experience itself, e.g. the language, tools, documentation? Is it something you would choose to do long-term?

I'll put a bounty out unless I get fantastic answers.