Yeah, kinda like saying I have a saw, hammer and level: what can I build?
Answer: darn near anything.
But I assume you're sort of asking about connectivity and how to make the best use of them together.
The first thing you want to do is connect them all up so you can use them when you're away from the iMac, which you'll be treating as your server. I have come to really like several apps that connect up to your Mac: Goodreader, Remote Viewer, Remoter VNC, SplashTop, iStat and Air Video. EyeTV is also great, but of little use unless you have the hardware to connect to an antenna. TouchPad, which works with SplashPad, allows you to use the iPhone as a, well, touch pad. And Printopia, to print to any printer connected to the iMac, and Phoneview, which gives you more control over connecting to your phone. To make best use of this stuff you've got to make your iMac accessible over the internet as well.
And try Telekinesis/iPhone Remote; it's in Google code or somesuch. It's free, and will give you an intro to various ways to connect and control your iMac from the other two.
Rob