So I'm torn on this. I like the tablet mockup with a table of contents on the side (though I actually tend to use the normal view on a tablet) but am a bit skeptical about the phone sized one.
The small issue: The floating contents button covers up a bit of content (not much obviously but anything will while scrolling) and has some oddities while using.

The bigger issue is fairly easy to fix: There is no way to escape out of the contents section (like an x or back) other then clicking a section once it's opened which was confusing when I wanted to drop back to the main list of condensed sections.

The biggest thing though is more of a gut feeling. It just feels... clunky. I think that if using the site myself I'd rather use the, already hidden, sections (pressing show/hide as needed) rather then the contents and save the extra click each time. Having to click over in the corner to get a table of contents seems a bit superfluous when the section list (with a show/hide) essentially acts as the same thing showing what is available.

Of course if you're in the middle of some text, those show/hide buttons are potentially a long-distance scroll away, whereas a floating button is always accessible... Generally the initial section should be initially expanded (which pushes all those sections down in the first place in most cases too); if it's not that's a bug in the current mockup. :)

What if, instead of just the contents button, it were a toolbar giving immediate access to search, contents, and settings?

-- brion