While I was at MacWorld this year, I worked my way up to a very crowded booth to see what all the fuss was about and ended up talking to the nice folks at GelaSkins. In addition to meeting some really great Canadians, I got to look at their product up close and was very impressed. Enough so that I bought several skins for me and for family as gifts.
Although their website talks about the skins as "protection" for you devices, I think we all just need to be honest and admit we are buying them because they are so cool. They may help you avoid a few scratches here and there, but the real reason to get them is to customize your stuff with some very creative and beautiful artwork.
Of course, being the geek that I am, I had to choose something with a robot in it. The architect in my also likes the symbolism and metaphor in this painting as well.
You should check out their selection at the GelaSkins web site. They have a huge variety of styles that will make it hard for anyone to pick just one. I went back to the booth three times before I finally chose the above image by Brian Despain for my matching iPhone and MacBook Air "upgrades".
Here are just a few examples of the range of images you have to choose from for iPhone 3G...
Note... you also get a matching "background" image for you device with every skin as well.