One of the hurdles you will be facing when building a Windows Phone 7 application which needs to integrate with SharePoint Server 2010 is the fact that WP7 does not support NTLM windows authentication. There are currently two different options:
- Integrate using Form Based Authentication
- Integrate using Microsoft ForeFront Unified Access Gateway (UAG)
For more information check out Security with SharePoint and Windows Phone Applications which is part of the SharePoint 2010 and Windows Phone 7 Training kit.
These are the two official ways of doing it but I recently found a third option where you can access your SharePoint site from Windows Phone 7 using basic authentication as outlined on Darrin Bishop’s blog.