Android applications are written in Java. You will need to know Java. I knew this before I started but knew very little Java. So whilst learning to develop Android applications I knew really I would be learning Java. I had hoped that the various Android development books would also be a nice introduction to Java along the way... but no.
I have been doing my Java learning in the background, using a book (see left) and a great set of video tutorials. The videos are even more suited to the aspiring Android developer because whilst learning Java you are also learning how to get the best out the Eclipse IDE which most Android development books/tutorials recommend. The only downside of the videos is that they are in SWF format. I spent a little bit of time converting them into something I could watch on my Advent Vega tablet.
The book (Beginning Programming with Java) is about to get a revision so it might be worth while you holding out for it. To be honest the video tutorials are in my opinion a better introduction to Java. Plus they are FREE!