I really like having the table of contents available at all times. Is there any reason we didn't explore using position fixed for the tablet table of contents to make it available at all times?
I've been thinking that'd be a great thing for us to do on the apps when we get to tablet optimization, too...
You'd have to make the list separately scrollable since the list may be pretty long on some pages and overflow the page. This should be pretty easy on modern versions of Safari, Android classic browser, Chrome, and Firefox, but I don't know if it'll cover some obscurer browsers. (Do we worry about Opera for tablet?)
Might also consider landscape versus portrait issues -- in landscape mode on even a 7" tablet there's plenty of horizontal space for a ToC but in portrait it's often a little cramped to always show a sidebar. I'd recommend having the presence of a sidebar depend on the available width (or make it slide-out when not wide enough).