For good, simple, updated information dealing with 2005 & 2006 purchases, here's a link to the Highlander Tax info from Toyota:

http://www.toyota.com/vehicles/2006/highlander/tax.html

For 2006 purchases, here is the certification letter in PDF form from the above site. Print it and keep it with your records for taxes due in 2007.

http://a230.g.akamai.net/7/230/2320/...ybrid_my06.pdf

I hope this helps clear up any questions if any.