Getting 403 Error when accessing lists.asmx

Question

I'm using this simple program:

http://www.codeproject.com/Articles/24244/Access-a-Forms-Based-SharePoint-Site-s-Web-Service

to get the services from my sharepoint 2007 site.

The authentication works fine, I receive the cookie but when try to get the list collection GetListCollection() I get an Exception:

The request failed with HTTP status 403: Forbidden

Any clues? Has anything to do with the _vti_bin web.config?

Thanks