Pilots N Paws was proactive on this question and took necessary steps to obtain clarification from the FAA.
"We had asked for and received clarification in order to best inform pilots on the status of the flights they so generously donate to help animals in need. It is quite clear under part 91 that GA pilots are not permitted to receive any monetary compensation for their rescue flights. However, due to our 501c3 status, it is acceptable for pilots to claim various portions of their flight expenses as charitable donations. We are posting this letter for you to use should you require it for your accounting purposes and specifically to make certain that everyone knows these flights are donated by pilots, there is no fee."
http://www.pilotsnpaws.org/forum/vie...php?f=3&t=6605
Letter is here and was dated October 2009:
http://www.pilotsnpaws.org/forum/dow...le.php?id=1402