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My best guess would be that the reason it's not available is that the document you're working on, hasn't had any new content added since you've updated to Lion.

i would think that you would have to create or edit a few documents after installing Lion for it to pick up Versions functionality, since there is no versions data for anything created on Snow Leopard.
 
The application has to support Versions for it to show up. I tried it out with the Preview app and a .pdf file. The only problem I had was the starfield background was all garbled with some of the Safari page I had open at the time.
 
The application has to support Versions for it to show up. I tried it out with the Preview app and a .pdf file. The only problem I had was the starfield background was all garbled with some of the Safari page I had open at the time.

TextEdit works as well.
The application you are using has to be written to use versions. Every application won't just be able to use it. Apple apps first, others will follow.
 
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