As far as I can see there doesn't seem to any mature environment for crossplatform development for iPhone & Android slate (but feel free to correct me).
I don't know Ipad, I must admit. Any reason why plain Java wouldn't cut it? (maybe underlying things like database (I am not too concerned with hardware))
I am thinking to try to make everything browser based, although that won't get around the d/b & h/w issues, unless there is JS framework that does so.
Any advice?
HTML5 (already mentioned) could be a good option.
Some other options are:
PhoneGap - an open-source cross-platform mobile development environment. It basically uses a stylized web-page and Javascript that is compiled into a native app on each platform.
Worklight (http://www.worklight.com)- a commerical product that does the same thing, but it requires a dedicated Worklight server on the backend. Worklight also allows you to target targets additional platforms (with same codebase) such as Yahoo Widgets, iGoogle, Vista gadgets, etc.