(Some themes are animated others aren't, perhaps if we had more people use non animated themes by default, it'd help)
Another option would be not to have a tab at all (a hovering tab is very annoying as it hides content and distracts from the site for a function which is not often used) - instead let customers include an unobtrusive help button in their actual page content wherever they wish which calls your js in order to display your overlay when clicked. You don't need an obnoxious tab.
I think what generally happens is that the team that handles customer communication does not have the dev resources to get someone to add UI elements to their page, so only bigger companies take advantage of "closeable olark" and click to open the chat window.
We also found that once people close the window, many get confused and have a hard time opening it again. (clearly not a problem for the "average HN reader", who might clear cookies and reload)
How would you want it to work? (i.e. we could do like, if query string contains hide-olark-for-30-days, we could just hide it for you, and if you wanted it back, we could give you an API command you could type into dev tools?
Our biggest concern (and we do allow hide-able chat boxes, is that once it's gone, many visitors actually want it back in the future on unpredictable intervals. So you can't just simple say "hide for 30 days")