It's a cool idea, but presented in a sidebar like that i think it's mostly a distraction. when you're writing you want less outside information, not more.
What if instead of a sidebar, you could get information about a word by putting your mouse over it and doing something? (does javascript let you trap rightclicks?)
The nice thing about a sidebar is you can investigate a broader scope of the document's semantic content, as opposed to just specific sections. (but yes, it does... and this might be a good way to narrow the context)
It's a cool idea, but presented in a sidebar like that i think it's mostly a distraction. when you're writing you want less outside information, not more.