All you need to complete this project is:
- a plastic smartphone cover that you want to customize (I used clear, but colored ones probably work well too!)
- some masking tape
- several colors of nail polish ( a good excuse to buy some wilder colors!)
- nail polish remover to clean up any messes
Here's what you do:
 use the masking tape to mask any sections of the cover you do not want to paint. In my case, I wanted the chrome edges on the phone to show, so I masked the sides of the cover
 Dab the nail polish colors on to the case in an attractive pattern
 Let dry for several hours. This is important, as the polish will be thicker than usual, and you want to be sure it won't mar or smear when it is not quite dry
Optional step  Put on a clear coat. I happened to have some acrylic varnish on hand, but it's not worth buying this for such a small project. You could also put a clear coat of nail polish on top
Once dry, add to your phone and admire!
I leafed through some art books to get inspiration for the color arrangements. Since the brushes are quite thick, don't plan a fine design. Something a bit abstract or impressionistic is best. I also found that the polish is too viscous to do much with drips a la Jackson Pollack.
If you get something you don't like, remove with the polish remover (acetone), or peel off when dry.
I'm rather pleased with this cover, and like the fact that it's all business-like from the front, and all party on the back. This happens to be a Blackberry Bold 9900 -- I would show you what it looks like on the phone, but I was using it to take pictures.
For gifting: you could also include the nail polish as part of the gift.