This message is produced by xdvipdfmx, called by XeTeX in order to produce PDF from the intermediate .xdv (extended dvi). While writing the PDF, xdvipdfmx dynamically allocates entities that translate into PDF objects in the output PDF file. The memory (RAM) should be properly deallocated once a PDF object has been written out and doesn't change any more. This fails somehow in older versions but doesn't affect the output PDF. In the upstream dvipdfmx, part of current TeXLive-2012, from which xdvipdfmx is derived, this bug has been fixed already. This warning message can be ignored safely.