Android id naming convention: lower case with underscore vs. camel case

I'm currently programming an application for the Android. Now what I found out is that you cannot place resource objects, say, an image in the drawable folder and name it like "myTestImage.jpg". This will give you a compiler error since camel case syntax is not allowed, so you'd have to rename it like "my_test_image.jpg".

But what about ids you define in the XML file. Say you have the following definition

<TextView android:id="@+id/myTextViewFirstname"
              android:layout_width="wrap_content"
              android:layout_height="wrap_content"
              android:text="Firstname" />

This is a valid definition, compiles and works just fine on my Android emulator although - as you see - I'm specifying the id in camel case syntax.

Now, the Android samples always use lower case and underscore. Is this just a naming convention to use lower case with underscore for the id's or may it cause problems on the real device?

Thx