While the viewstate control is a hidden field it is still optimal to move it to the bottom of the page or to disable it all together. Personally, if your viewstate isn't more then 5K, you should be ok from needing to move it to the bottom of the page.
Be warned though, google places a bit of emphasis on where X keyword appears in your copy so, if you have 100K of viewstate (hell even 5K), X keyword is going to appear much farther down the page than if you move your viewstate.
The "best practice", of course, would be to move the viewstate to the bottom of the page and have your content be the first chunk of data directly after your <body> tag, followed by your menu, and then your viewstate.
(BTW, you might want to check out Christian and Jamie's book: Professional SEO
with ASP.NET, follow the link in my profile if you are interested.)
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