If you get the following error:
ProtocolError occured trying to complete the request. The server returned a status code of : Forbidden and the status description is : "Forbidden"
Problem: The machine account or app pool identity is trying to access the feed and cannot.
Solution:

• Enbable anonymous access to the RSS feed. This may or may not be allowed in your environment.
• If you are using Kerberos...(i.e. have already configured for delegation)
  • Go to Central Admin
  • Application Management
  • In Application Security, click Authentication Providers
  • change to Negotiate (Kerberos)

Ensure that your app pool identity is configured for delegation. Heres a good article, http://blogs.msdn.com/markarend/archive/2006/10/03/RSS-Viewer-web-part-and-authenticated-feeds.aspx

At crawl time document ACLs are stored.  If you add users to MOSS individually a crawl is required for any updated ACLs to be reflected in the search results. I you add users to an existing AD group a new crawl is not required.

Keep in mind, however, that ACL changes are stored during incremental crawls.