Ok this is driving me nuts!
It defies logic that this is not possible - I recently bought a lonely planet app which was around 500 mb so I downloaded it on on laptop.
However I cannot transfer the app to my iPad unless I sync my iPad to my laptop (and in turn lose all my apps on my iPad which are not on my laptop)
Transferring music by dragging and dropping is fairly straight forward but it's impossible to do the same with an app???
It makes no sense.
Unless of course I am missing something?