Stumbled over this cool webpost from the Dynamics NAV Blog. It is about Webservices in version 5.x. It includes a nice project for at COM object in C#, to communicate with the webservice. It is written by Kris Rafnsson.
I have not installed the project yet, but will start looking at it right away, as i need this for another project. I will post my comments on it then.
For now take a look at:
http://blogs.msdn.com/nav/archive/2008/04/15/using-web-services-to-access-microsoft-dynamics-nav-5-0.aspx
and here the c# project:
http://code.msdn.microsoft.com/nav/Release/ProjectReleases.aspx?ReleaseId=896