In Japan, in addition to the current White and Black models, the DSi was recently released in three brand new colors: Pink, Lime Green, and Metallic Blue.
If you want one of these new colors, you could always import, but this complicates things - language barrier, different Shop Channel, and the region lock on upcoming DSi-only titles for instance. There is always the chance that they will be released in your territory, though it is certainly not guaranteed, and if you already own a DSi, it's possible you'll be losing money on the deal.
There is another option however, one which can give you your desired color scheme, add protection to your system, and give a feeling of individuality.
Hori of Japan has a line of protect cases that are available in the same color schemes as the Japanese models, and looks to be your best bet for prettying up your new DSi. These protect cases are solid, not like the rubber gloves you may have seen on sites like eBay or Amazon. The protect cases cover the outside of the system, protecting it from dings and scratches, and leaves openings for all the ports so that your gaming is not limited in the slightest while using it. They also add very little to the size of your system, so that your DSi will still fit in its case.
The White and Black cases are already available on import sites such as play-asia.com, but Hori's official website lists the Pink, Lime and Metallic Blue protect cases with a release date sometime in May. Other protect cases are also available from Hori including clear, clear black, clear blue, clear pink, and one modeled for the upcoming Dragon Quest IX.