I almost hate to bring this up. To people who have been around here a while I must sound like an X10 evangelist 
!
However, in case you haven't considered it, you should be aware with what you can do with Arduino and X10. And particularly, if you live in N. America, I don't think you'll find a more economical solution to control lights and appliances either through RF or by sending the signals down the power line (which is essentially wireless). In addition, you are using UL approved devices and not mucking with mains voltage, and putting DIY boxes in your walls and ceilings.
Here is a diagram I made to get an idea of costs . . .
With an $8 box (PSC05) the Arduino can send commands over the power line to turn on lights and appliances. It can also receive commands that have been put on the power line by other devices. With a $6 box (CM17A) the Arduino can also send commands over RF.
I have a little box on my desk that has an Arduino in it that "controls" all the lights in my house. Control can mean simply turning on and off lights at specific times, or more complex things like if it's a certain time, and motion is detected, do this, and this . . .
In your case, that little box can be connected to a web interface. Here is a guy who is doing that:
lookout: June 2010
There are libraries that you can get on the Playground that interface with the devices mentioned above.
For additional info, I have a blog that shows what I have working. Arduino Collection: Home Automation
Believe me, I'm not trying to sell you on X10. I can give you it's disadvantages too. However, when I hear that your plan involves multiple Arduinos, Xbees, and SSR's I can't help thinking how the limitations of cost will force you into a smaller system than you would like.
At least it's worth checking out if you haven't.
Have fun building your system! 
John