This is a general warning, not particularly related to the Tweet This plug-in but I have to put it somewhere! It's about a firm called Th8.us - an URL shortener. If you use this, when people click on it it produces adverts on top of your website. If you don't mind that, fair enough, but I do very definitely mind.
I have changed my url shortener to bit-ly which I use anyway. It's a drop-down list in the Tweet This settings. People should know about this!