xBuser, if you haven't left yet, and have time, and don't need to take more than one passenger, take out your rear seat! Here is an insanely well detailed take-apart of a Scion interior, with seat removal instructions. I did this when I went to Burning Man, and found it relatively easy, and the seats are heavy enough that it will make a difference in MPG. (warning: lots of high-res pics below if you have a slow connection it will be slooow) You just need a socket wrench and maybe 20 mins. of time. You'll be amazed at the room it opens up back there if you ever need to use the xB for even more hauling duty - remember, it's much more aero to carry things inside than on the roof!
ICIXSound.COM Forums - Scion xB Walkthrough and stuff...56k = no