Almost all the documentation on android development is focused on Java, and eclipse. While I'm not a big fan of Eclipse as an IDE (bloat); it is what the lot use, so it is what I use.
Where you'll find hardship with ruby is when you have to do something that is atypical.. something that is purely android. It will save you time in the beginning, however as with all high level languages, it will end up costing you more when you need to get "dirty"... The net result will be the same.
I program ruby/rails during my day job and android for my "unpaid". Both are good languages, but there are strings attached with any language.
Keep in mind also that the Android platform is Java supported.. In this you know if there are new features in OS 5..6..n then you can be sure there is a way to access them in Java. Ruby might be quick to integrate them, but then it depends on the developers (us) of Ruby. Also keep in mind that on some older devices 1.x..2.x there may be things that run slow when passed through ruby. (I don't know how it's interpreter works)..
Last note. If you learn Ruby for android, and you apply for a job programming Android, I'm betting they will expect Eclipse/Java not xxx/Ruby
