I am typing this as I am finally connected in shell to my Android phone. The prompt reminds me that it's based on the Linux kernel (it's free), the Dalvik virtual machine (it's free), and free libraries. Millions of Android devices are shipped every day, each one is a Linux system. Today, it's phone. Soon, it will be tablets: Android 3.0 (coming out at the end of the year) will finally be very suitable for tablets.