If you use the addthis button with default instructions of the addthis.com site, you'll receive this behavior: the page appears to load forever. Checking your cookies shows that certain cookies were not allowed. Patch this behavior by using the tip published here: http://www.addthis.com/forum/viewtopic.php?f=5&t=14847
A fix is on the previous page
PS: if this "eternal loading" does not happen for you, don't be alarmed. IE7 sometimes does, sometimes does not show this behavior, depending on your cookie settings and depending on whether you loaded the previous page first, whether the page was refreshed (i.e., after an empty cache) or when you have or have not installed flash and what version of it.
PPS: Also possible that at some point, this behavior disappears because at addthis.com they might roll-out a new version of the javascript.That's why on this page I use a copy of the original script to show this behavior.