First the license plate, now the cell phone.
This is actually really easy. If you have T-Mobile (I know you can do it with other services too), your phone automatically has an email address. The address is in the form: <phone_number>@tmomail.net. So, if your phone number is (801) 867-5309 (that's the number I hand out to annoying cashiers (I hope know where that number comes from)), then your phone's email address would be: email@example.com.
Ok. Now the fun part. Get your favorite image, in this case, it's the Ubuntu logo, resize it to fit your cell phone's screen (in my case, it's 120x120 pixels), and send the logo to your phone's email address with the image as an attachment. The image can be jpg, jpeg, gif, png, bmp or wbmp. After it's sent, you'll get an alert of a new picture message. Download the picture, save it in your phone, and change your wallpaper.
So, as you can see, I'm wandering around Utah with "UBUNTU" on my license plate, and the logo on my phone. That's what I call a passion! And, if someone needs to borrow my phone, I'm advocating the Great Distro Ubuntu.