To use multiple arduinos together, I2C library is a very useful source. To make it more clear I created a simple tutorial for arduino web site. It explains how to make physical connection between arduinos by using sketches (using fritzig) and text. Also, it includes some code samples.
This is an ongoing project and it is a collaborative work with Tom Igoe.
Here is the link for my blog post: http://www.mustafab.com/blog/?p=587