The official Android Developer website really is THE perfect place for anything related to Java development for Android (*), and it is regularly updated !
You should start with the Android Basics in the Android Dev Guide, that will give you an initial overview of the platform.
Then, you can follow the Tutorials in the Developers Resources, after what you would be able to look at the Sample Code in the same place.
Personally, I also did a complete survey of the Developer Blog that is full of good idea. I am now following it through RSS feeds, but some of those good technical advice can also be found in the Technical Resources->Articles section, in the Common Tasks and in various FAQs.
Last but not least, if you like, you could also follow some of the Google I/O conferences and other related interviews on the Videos section.
I also love to look twice a month at the charts of Android Versions showing how new platform API are spreading, which can help you choose the API level you want to be compatible with (edit: chart updated in April 2011) :
Since the beginning (December 2009), I’ve decided to stick to Android >=2.0 (API level 5) and now Android 2.1+ represents more than 80% of the active devices (edit: now more than 90% at the time of the chart upload).