The builtin report viewer control from MS requires ViewState to be on to work. You'll get told this if you have your ViewState turned off (as it should be). Now, you can go into your page and EnableViewState='true' like a good boy, but remember that if you are using a MasterPage you must explicitly enable view state for that too...Why? Both are just controls and of course you can have view state on for some controls and off for others. That, plus the granularity of control over this beast of ViewState is ridiculous.
I never will understand why control developers assume everyone uses or LIKES to use ViewState.