Well, I have firsthand experience regarding this and I can safely say that you can get an iphone now and still be able to get a phone at the discounted price shortly after that (without waiting 2 years).
My plan was eligible for an upgrade september 4 and i bought the iphone july 20th and activated it (this starts a new 2 year contact starting from july 20th). I placed an order for a blackberry curve on september 4th and just received it yesterday, for their discounted price, which I will sell on ebay. The only catch is that I started a new contract due to the "upgrade". So now instead of having my contract expire on july 20, 2009, it expires september 4th, 2009.
I would say go for the upgrade to the iphone now if you dont mind starting a new contract whenever you "upgrade" from the iphone.
was it worth it to me? hell yes.