This project was designed to get you used to working with cheap and simple RF modules while providing a cool, practical application. There is much room for improvement here. For instance, if this were a long-term project, much code optimization would need to be done to have the processor "sleep." Also, for alerting purposes, perhaps an LED or some audible alert could be added.
In O'Reilly's "Arduino Cookbook," in recipe 14.1 Sending Messages Using Low-Cost Wireless Modules, there is much more information about using these RF modules.
You can get the book here at the Maker Shed.
For another mailbox alert system, check out Matt Richardson's Snail Mail Push Alerts project.
And for lots more Arduino, take a gander at the Make: Arduino page.