Nintendo of America is still evaluating whether or not to release a pink version of the Nintendo DS handheld, according to the Nintendogs website. If you watch Brielles video diary on the Nintendogs website you will see her using a pink Nintendo DS. In the FAQ section of the site they respond to the question "Whats up with Brielles pink Nintendo DS?" with:
The pink Nintendo DS system you see Brielle using is not currently available in North America, but if you want to see this color come to the US. you should let us know. Voice your opinion at www.NintendoKennelClub.com or forums.nintendo.com.

Thanks to Kotaku.com.