Marshall - 3:02 am on Feb 29, 2012 (gmt 0)
Hi lsargent and welcome to WebmasterWorld.
Whether you use (for example)
<div itemscope itemtype="http://data-vocabulary.org/Product"> or
<div itemscope itemtype="http://schema.org/Product">
you do not need to place xmlns:schema="http://schema.org/" in the <html> tag. It is placed in the <div> tag as I have shown above. I cannot say I have ever seen the schema in the <html> tag. Also, I have put organizational microdata, a.k.a. Rich Snippets on several sites for both as results in SERPs and in use with Google+, and never added that.
After you get the snippet together, test it with Google's Rich Snippet Testing Tool [google.com] But do note that just because you have them, does not mean Google, or any other SE, will include them in SERPs. Google will, however, recognize it for when a person uses the Google+ button.
Also, remember that Facebook will not recognize the snippet as they use Open Graph Protocol (OGP [ogp.me]). Something you may want to consider if you have a Share/Like on facebook link. For that, you do have to add a schema to the <html> tag -
Note: in the above schema, there is no space between the : and o. I had to put it there as the forum kept making a :o