I dont think so.
Android is a complete software stack- it comes with an OS(Linux Kernel), middleware, application library and framework classes. Any OEM can manufacture devices that can "run" Android.I have even heard of Android based NetBooks
I'd like to know if it is true that Android only works in Google devices, like G1.
If so, isn't against Java WORA concept (write once runs everywhere)?
Absolutely not.... Android can run on any device on which you can compile Android OS source code and there are device drivers for the hardware you want to use and Android SDK supports (screen, keyboard, WiFi, Bluetooth, mobile network layer, etc.).
Android is not exactly Java, therefore WORA concept does not apply. Have in mind that Android applications are written in Java using Android SDK which run on Dalvik VM (Virtual Machine) and not on ubiquitious JVM (Java Virtual Machine), which has been supported on many operating systems and hardware platforms. Also, Android SDK is not identical to Java 5 SDK. Rather some parts have been ported.
All in all, you can use Java skills to write Android applications and there's some (substantial) effort required to port existing Java (SE/ME) applications to make them run on Android devices.