looking at the tech specs [1] you can only search a single index at a time which is pretty limiting. hopefully just a limit of the preview.
I am a Program Manager for the Azure Search team. I am glad to hear you think our service looks promising. You are right about searching across indexes. This is something we heard often from a number of customers we worked with before today's annoucement. There are often ways of working around this, for example by merging content into a single index, but this is obviously not workable for everyone, so you are also correct, that this is really just a matter of time.