hi,
I setup a customized service on my addthis button for my blog on http://rottencapital.tumblr.com.
It works well on the subpages and also on the indexpage the JavaScript in the addthis_configuration is correct, with the URL of the post, as well as the title.
However, when multiple addthis buttons are on one page, even if the JavaScript configuration for each Addthis setup is correct, however it somehow sends only the url and title of the last one to the flattr service.
IMHO this is a bug in Addthis.