I have an '04, delivered in Oct '03. It now has every gadget I want and is as luxurious as it can get. I plan to keep the car in the family until the paint falls off and Toyota no longer can fix it.
Every day I would have waited would be one day less of enjoyment - one additional day paying more for gas and emitting more crap into the environment.
And today, I can sell my '04, which has 27k miles on it, for slightly more than I paid for it almost two years ago.
The Prius is unquestionably the smartest car purchase on today's market, and the only unintelligent buying decision is to defer the transaction.