It takes a long time rummaging around the Toyota web site to find anything related to the question. Not a good sign. And granted, this is the 2009 web site, not the 2008. But I finally arrived at
http://www.toyota.com/sienna/options.html
which says the tow rating is 3500 pounds. But there is the inevitable asterisk (Toyota owners often deny its existence) saying
"The maximum you can tow depends on the total weight of any cargo, occupants and available equipment."
After much MUCH discussion on this forum, it seems that this warning translates to "you can tow 3500 pounds of trailer in the low altitude flatlands, provided you don't put any options, occupants (other than the driver), or cargo in your Sienna."
Having said that, let me also say there are Sienna owners on this board who will certainly relate their experiences, and some of them are good. As always, the first question is "Where do you intend to tow?" And let me also say that your desire to find the GCWR, and HarveyRV's comments, are a very good way to proceed.
Bill