Arduino vs. Wiring?

The Wiring website nicely summarizes the differences between the Wiring board and the Arduino. What isn’t clear is how closely compatible are they? Will a program written for the Arduino run on the Wiring unmodified? Minimally modified?

We’ve tried to keep things compatible, so the basics (like reading and writing pins, serial communication, etc.) should work unmodified. Some things that work on Wiring aren’t yet supported on Arduino because of hardware limitations of the ATmega8 chip onboard (which we use to keep the prices down and because it’s easy to replace). Other things are in the process of being ported. But they should be pretty compatible, yes.