I'm sure others can tell you in more detail what differences there are, but from what I've heard so far I believe there aren't any major changes to the 2006 models (in terms of the hybrid systems)... so I don't know that you'll really gain anything in that respect.

Also, regarding the tax incentive issue, the federal and most state incentives I've heard of apply to the year (date) that the vehicle is purchased. So if you buy a 2006 model in 2005, you should qualify. So no real difference there either.

It looks like it boils down to the discount and immediate availability versus the premium package options.