Multi OS Programming (including PC and Android)

[goatjugsoup]

I'm doing a student project where I would be looking to make a desktop application that can run on multiple operating systems. Extra credit for also getting it to run on a smartphone.
I've got a good background knowledge of a few programming languages but no idea where to start for getting an application to work on multiple operating systems.

Specific questions I have are

What language should this be programmed in to facilitate the least amount of changes necessary for other OS compatibility?
How could I test a smartphone app without a smart phone (possible even?)?

General tips would also be appreciated as there are no doubt questions I should ask that I wouldn't even think of at this stage.

Thanks in advance

[Chris]

www.appcelerator.com/products/titanium-cross-platform-application-development/

Might be of interest.