I would avoid buying a redesigned model of ANY car (i.e. a 2003 Sienna) for one or two model years. This gives the manufacturer time to iron out the wrinkles and refine the design.
With modern computer aided design and manufacturing techniques, this isn't quite the issue that it once was. Nonetheless, let other buyers be the guinea pigs. Toyota doesn't want to lose any 2002 Sienna sales so I don't think they'll be in a hury to unleash the 2003 model.
Buy a proven design now. If you like the new Sienna when it arrives, trade-in for one in 2005.