Anyone may attend the National Convention, but only members may "compete" in the National Contest.
From the "Eligibility" section of the IMPS/USA Competition handbook:
"Any current member of IPMS/USA or any other national branch of IPMS is eligible to enter any number of models in the National Contest. A member may also serve as a "proxy" to enter models for a fellow member who is not attending the convention..."
All other information regarding the 2005 National Convention can be found either on the official web site, http://www.ipmsusa2005.org/ , or through the links on that page. The "Contest" link contains all of the information for that portion of the convention.