When I bought my Sienna they had only been out for a month or so. I was told it had a 3500 towing capacity and that no "towing package" was available to up that (not that I needed it higher).
But, according to the Toyota web site (
http://www.toyota.com/sienna/models.html), all models of the Sienna come standard with a towing package that is rated to 3500 pounds.