You can develop an HTML5/CSS3/Javascript app, then wrap it in PhoneGap to make it native and put it on the App Store (or Android Market...), as well as publish it on the web.
The added benefit is PhoneGap allows you to do additional things such as parse the iPhone contacts and access the local hardware.
It's great in combination with jQuery Mobile, but you must be very caution to really streamline your images, CSS, and Javascript. Performance is a major issue.
Here are some great links I found to improve PhoneGap / Sencha / Mobile Web App performance.
See this to learn more about Mobile Frameworks: http://www.markus-falk.com/mobile-frameworks-comparison-chart/