I do this too. I'm convinced it's a vestigial habit of the "old" web, where it was very common to have a crazy img based background and text that you couldn't read. I found myself highlighting text on the web all the time just so I could get better contrast.
It's possible myspace will create a whole new generation of obsessive highlighters.
Annoying and redundant. I often find myself clicking on a page to either force focus to that window, or to select text to act as a highlight so I can find my place easily later on.
This means any accidental doubleclicks are launching new tabs or windows that I don't want.
If I want a definition of a word, I have more global, convenient ways of handling that.
Now, if instead double-clicking did something like spawn a topic search through NYT or news.google.com or something, without the hassle of underlining every word, I can imagine that would at least seem useful.
I dislike it, very much, mostly because it is intrusive (pop up). Its behavior violates the principle of least surprise, since most websites and applications don't do that.
If it was an inline popup box in the margin, I would probably like it.
An interesting idea, stupidly mangled by the corporate process. If they manage to make it useful that would be nice. But I suspect it's just ploy to boost pageviews, and will be measured solely on that.
You can only do one word at a time. I can look up "republic" and "day", but not the phrase "republic day".
The click time-window is long, about 800msecs. Lots of accidental pvs there.
It's a terrible idea. I bet someone had to come up with a feature to justify their IT job and created this monster. This, along with the PR "business" articles in the fashion section, made me lose the most respect for nytimes.
I was looking for the same thing yesterday (if I understand what you mean about a vocab tool). I settled on the answers.com extension (http://www.answers.com/main/firefox_plugins.jsp). I didn't look around very hard so maybe there is something better out there.
Anyhow, I have already have the answers.com plugin for Firefox, which brings up that overlay on command.