Incentives and rebates are taking place now. The 2009's are due in June/July and there will be more pressure on the 2008's if you wait until then. However, you will run the risk of not getting the exact 2008 that you want.
If you look at the pricing now on the 2008, http://www.edmunds.com/new/2008/infinit
i... you can get the FX35 for about $600 under invoice. That's a pretty good deal. You should configure the Edmunds TMV for your area and the exact FX35 you want.
You should also read the Infiniti FX35 Lease Questions discussion at http://townhall-talk.edmunds.com/direct
/... where people are discussing the lease deals they are getting.