The following link was mentioned by Manoj Nagpal on the Aus-DotNet mailing list this afternoon. It's a must read for all ASP.NET developers. It's a very enlightening article on ViewState.
http://weblogs.asp.net/infinitiesloop/archive/2006/08/03/Truly-Understanding-Viewstate.aspx
It's not often that I simply drop a link on my blog but this one is well worth it.