I use Adobe AIR to build applications for desktop or mobile. Using AIR allows a cross-platform (i.e. both Windows and Mac, both Android, Blackberry, and iOS) application with one code base.
A blog post that sums up why AIR is a great choice for cross platform development.
If you want a real lot of interactivity, and you are not incredibly worried about visitors on iOS (iPhone or iPad), then we should talk about the option of doing the site or part of it in Flex.