That codeplex project is by Stephane Crozatier, who has this to say about it on his blog:
I’m not on the Windows Phone team. This is not the official control, and i don’t know if they have plans for such controls in the future. The developer FAQ seems to suggest something may be coming down the road.
This is not production code. I am using it as a learning experience for Silverlight, which i’ve started less than 2 weeks ago. I’m still learning about the technology and writing controls : there are lots of areas for improvements in the current implementation (quality and robustness, templates and customization, resource usage, fonts, themes, …).
This is a pet project i’m working on during my free time, just for fun. For now there’s no plan to turn it into something more than a pet project, although i’d like to continue the effort and perhaps build something i will call the Pivot control.
Stay tuned for more, and remember : this is by no means a Microsoft supported and official project, i’m just a guy trying to learn Silverlight on the Phone. Please use the official controls if/when they are released.