I found some great themes put together by my brethren over in the Cydia store for the iPhone/iPod touch.  To get these, you have to "jailbreak" your phone.  Which, thanks to legislation and our friends at jailbreakme.com, it's just a browser click away.  But first, connect your iDevice to your computer and make a good backup.

So, now that we have a good backup, we need to install the Cydia App store.  So, launch Safari on your iPhone, click on the "Free" button next to the Cydia App, then click on the "Install", just like you would in the Apple Store.  Once installed, we need to go into the Cydia App Store, search fo and install "Winterboard".

Once Winterboard is installed and your phone has been rebooted, go back into the Cydia store and search for "Winterboard Mason", and look through the themes.  Most have preview images.  My personal favorite is "Free Mason" by Jason Huber.

Once the theme has been downloaded, go into Winterboard, select "Themes", select your theme and exit out.  Your phone will "respring", which is a quick restart; and you're good to go.