I did saw that 1.99% APR from bmw. Very tempting. I think the cheapest you can get is 3.99% from Pentagon Federal Credit union or if you have a local bank that offers even lower. I used them to finance mine:
PedFed
Since you've just bought the bmw 6 months ago, I would suggest you to enjoy the car for a little longer. Drive the BMW for a while and check our forums everyday to see any update on the Z. Once Nissan release the 2010 model year toward the end of the year, you can either a. Get 2010 model or b. get 2009 model for more discount.
You're correct, the dealers are asking very high interest rates. My local ones are asking anything from 5.99 to 6.99.
Good luck with your Z hunt. Try to absorb as much info as possible from our forum before buying your 370Z.