I still have the problem with the very latest build (built on 11/02). The bug seems to be a little bit more complex than mentioned initially on the mailing list. It's not working only in this scenario:

-1. Launch Xfce

-2. Insert immediately a removable disk (the icon appears on the desktop and the disk is mounted according to "mount" in a terminal)

-3. Double click on the new icon. I get the following error in console #1: 
(xfdesktop:5632): CRITICAL **: xfdesktop_file_icon_activated: assertion 'info' failed. According to the source code, xfdesktop returns at the top of xfdesktop_file_icon_activated.

Note that if xfdesktop is started AFTER the insertion of the removable device, then it's working perfectly well and the message mentioned above doesn't appear.

It seems to be a problem of initialization of icon.