Thanks again for your reply. I'll have a look at API.aspx.
You may be right about sending all params. I'll find out when I get past the authentication error.
For authentication, I'm looking at:
CatalystCRMWebservice Web Service
I don't see any header there, just <Contact_CampaignListListUpdateInsert>, which contains properties for username and password. However, it's my understanding those are the customer's username and password, not my authentication. Am I missing something?