I have yet to create a custom background for my Twitter page. However being on my second week of really diving into Twitter, I have noticed other bloggers and big time Twitter folks take the time to customize their background.
There seems to be a basic convention to it, and I have to admit it makes it look more professional/more information/and generally more appealing.
Here are some great resource links that I found while researching this:
There are quite a few more out there. I am thinking I will start with something fairly basic using the PowerPoint methedology, and time permitting work on something a little more flashy via Photoshop.