You need to choose to save the animation sequence as images. Pick png or tif as the file format (tga may work also). Those save the background transparent, as long as there's nothing in the background at all.

Artwork and 3DToons items, create the perfect place for you toon and other figures!

http://www.renderosity.com/mod/bcs/index.php?vendor=23722

Due to the childish TOS changes, I'm not allowed to link to my other products outside of Rendo anymore :(

Food for thought.....
https://www.youtube.com/watch?v=pYZw0dfLmLk